18 files changed, 1101 insertions, 184 deletions

diff --git a/crates/assists/src/assist_config.rs b/crates/assists/src/assist_config.rs
index adf02edab..b24527ec4 100644
--- a/crates/assists/src/assist_config.rs
+++ b/crates/assists/src/assist_config.rs
@@ -4,6 +4,8 @@
 //! module, and we use to statically check that we only produce snippet
 //! assists if we are allowed to.
 
+use hir::PrefixKind;
+
 use crate::{utils::MergeBehaviour, AssistKind};
 
 #[derive(Clone, Debug, PartialEq, Eq)]
@@ -37,10 +39,11 @@ impl Default for AssistConfig {
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub struct InsertUseConfig {
     pub merge: Option<MergeBehaviour>,
+    pub prefix_kind: PrefixKind,
 }
 
 impl Default for InsertUseConfig {
     fn default() -> Self {
-        InsertUseConfig { merge: Some(MergeBehaviour::Full) }
+        InsertUseConfig { merge: Some(MergeBehaviour::Full), prefix_kind: PrefixKind::Plain }
     }
 }
diff --git a/crates/assists/src/ast_transform.rs b/crates/assists/src/ast_transform.rs
index 835da3bb2..4307e0191 100644
--- a/crates/assists/src/ast_transform.rs
+++ b/crates/assists/src/ast_transform.rs
@@ -5,12 +5,13 @@ use hir::{HirDisplay, PathResolution, SemanticsScope};
 use syntax::{
     algo::SyntaxRewriter,
     ast::{self, AstNode},
+    SyntaxNode,
 };
 
 pub fn apply<'a, N: AstNode>(transformer: &dyn AstTransform<'a>, node: N) -> N {
     SyntaxRewriter::from_fn(|element| match element {
         syntax::SyntaxElement::Node(n) => {
-            let replacement = transformer.get_substitution(&n)?;
+            let replacement = transformer.get_substitution(&n, transformer)?;
             Some(replacement.into())
         }
         _ => None,
@@ -47,32 +48,35 @@ pub fn apply<'a, N: AstNode>(transformer: &dyn AstTransform<'a>, node: N) -> N {
 /// We'd want to somehow express this concept simpler, but so far nobody got to
 /// simplifying this!
 pub trait AstTransform<'a> {
-    fn get_substitution(&self, node: &syntax::SyntaxNode) -> Option<syntax::SyntaxNode>;
+    fn get_substitution(
+        &self,
+        node: &SyntaxNode,
+        recur: &dyn AstTransform<'a>,
+    ) -> Option<SyntaxNode>;
 
-    fn chain_before(self, other: Box<dyn AstTransform<'a> + 'a>) -> Box<dyn AstTransform<'a> + 'a>;
     fn or<T: AstTransform<'a> + 'a>(self, other: T) -> Box<dyn AstTransform<'a> + 'a>
     where
         Self: Sized + 'a,
     {
-        self.chain_before(Box::new(other))
+        Box::new(Or(Box::new(self), Box::new(other)))
     }
 }
 
-struct NullTransformer;
+struct Or<'a>(Box<dyn AstTransform<'a> + 'a>, Box<dyn AstTransform<'a> + 'a>);
 
-impl<'a> AstTransform<'a> for NullTransformer {
-    fn get_substitution(&self, _node: &syntax::SyntaxNode) -> Option<syntax::SyntaxNode> {
-        None
-    }
-    fn chain_before(self, other: Box<dyn AstTransform<'a> + 'a>) -> Box<dyn AstTransform<'a> + 'a> {
-        other
+impl<'a> AstTransform<'a> for Or<'a> {
+    fn get_substitution(
+        &self,
+        node: &SyntaxNode,
+        recur: &dyn AstTransform<'a>,
+    ) -> Option<SyntaxNode> {
+        self.0.get_substitution(node, recur).or_else(|| self.1.get_substitution(node, recur))
     }
 }
 
 pub struct SubstituteTypeParams<'a> {
     source_scope: &'a SemanticsScope<'a>,
     substs: FxHashMap<hir::TypeParam, ast::Type>,
-    previous: Box<dyn AstTransform<'a> + 'a>,
 }
 
 impl<'a> SubstituteTypeParams<'a> {
@@ -111,11 +115,7 @@ impl<'a> SubstituteTypeParams<'a> {
                 }
             })
             .collect();
-        return SubstituteTypeParams {
-            source_scope,
-            substs: substs_by_param,
-            previous: Box::new(NullTransformer),
-        };
+        return SubstituteTypeParams { source_scope, substs: substs_by_param };
 
         // FIXME: It would probably be nicer if we could get this via HIR (i.e. get the
         // trait ref, and then go from the types in the substs back to the syntax).
@@ -140,7 +140,14 @@ impl<'a> SubstituteTypeParams<'a> {
             Some(result)
         }
     }
-    fn get_substitution_inner(&self, node: &syntax::SyntaxNode) -> Option<syntax::SyntaxNode> {
+}
+
+impl<'a> AstTransform<'a> for SubstituteTypeParams<'a> {
+    fn get_substitution(
+        &self,
+        node: &SyntaxNode,
+        _recur: &dyn AstTransform<'a>,
+    ) -> Option<SyntaxNode> {
         let type_ref = ast::Type::cast(node.clone())?;
         let path = match &type_ref {
             ast::Type::PathType(path_type) => path_type.path()?,
@@ -154,27 +161,23 @@ impl<'a> SubstituteTypeParams<'a> {
     }
 }
 
-impl<'a> AstTransform<'a> for SubstituteTypeParams<'a> {
-    fn get_substitution(&self, node: &syntax::SyntaxNode) -> Option<syntax::SyntaxNode> {
-        self.get_substitution_inner(node).or_else(|| self.previous.get_substitution(node))
-    }
-    fn chain_before(self, other: Box<dyn AstTransform<'a> + 'a>) -> Box<dyn AstTransform<'a> + 'a> {
-        Box::new(SubstituteTypeParams { previous: other, ..self })
-    }
-}
-
 pub struct QualifyPaths<'a> {
     target_scope: &'a SemanticsScope<'a>,
     source_scope: &'a SemanticsScope<'a>,
-    previous: Box<dyn AstTransform<'a> + 'a>,
 }
 
 impl<'a> QualifyPaths<'a> {
     pub fn new(target_scope: &'a SemanticsScope<'a>, source_scope: &'a SemanticsScope<'a>) -> Self {
-        Self { target_scope, source_scope, previous: Box::new(NullTransformer) }
+        Self { target_scope, source_scope }
     }
+}
 
-    fn get_substitution_inner(&self, node: &syntax::SyntaxNode) -> Option<syntax::SyntaxNode> {
+impl<'a> AstTransform<'a> for QualifyPaths<'a> {
+    fn get_substitution(
+        &self,
+        node: &SyntaxNode,
+        recur: &dyn AstTransform<'a>,
+    ) -> Option<SyntaxNode> {
         // FIXME handle value ns?
         let from = self.target_scope.module()?;
         let p = ast::Path::cast(node.clone())?;
@@ -191,7 +194,7 @@ impl<'a> QualifyPaths<'a> {
                 let type_args = p
                     .segment()
                     .and_then(|s| s.generic_arg_list())
-                    .map(|arg_list| apply(self, arg_list));
+                    .map(|arg_list| apply(recur, arg_list));
                 if let Some(type_args) = type_args {
                     let last_segment = path.segment().unwrap();
                     path = path.with_segment(last_segment.with_generic_args(type_args))
@@ -208,15 +211,6 @@ impl<'a> QualifyPaths<'a> {
     }
 }
 
-impl<'a> AstTransform<'a> for QualifyPaths<'a> {
-    fn get_substitution(&self, node: &syntax::SyntaxNode) -> Option<syntax::SyntaxNode> {
-        self.get_substitution_inner(node).or_else(|| self.previous.get_substitution(node))
-    }
-    fn chain_before(self, other: Box<dyn AstTransform<'a> + 'a>) -> Box<dyn AstTransform<'a> + 'a> {
-        Box::new(QualifyPaths { previous: other, ..self })
-    }
-}
-
 pub(crate) fn path_to_ast(path: hir::ModPath) -> ast::Path {
     let parse = ast::SourceFile::parse(&path.to_string());
     parse
diff --git a/crates/assists/src/handlers/auto_import.rs b/crates/assists/src/handlers/auto_import.rs
index fa524ffd9..357ff6392 100644
--- a/crates/assists/src/handlers/auto_import.rs
+++ b/crates/assists/src/handlers/auto_import.rs
@@ -191,12 +191,16 @@ impl AutoImportAssets {
                 _ => Some(candidate),
             })
             .filter_map(|candidate| match candidate {
-                Either::Left(module_def) => {
-                    self.module_with_name_to_import.find_use_path_prefixed(db, module_def)
-                }
-                Either::Right(macro_def) => {
-                    self.module_with_name_to_import.find_use_path_prefixed(db, macro_def)
-                }
+                Either::Left(module_def) => self.module_with_name_to_import.find_use_path_prefixed(
+                    db,
+                    module_def,
+                    ctx.config.insert_use.prefix_kind,
+                ),
+                Either::Right(macro_def) => self.module_with_name_to_import.find_use_path_prefixed(
+                    db,
+                    macro_def,
+                    ctx.config.insert_use.prefix_kind,
+                ),
             })
             .filter(|use_path| !use_path.segments.is_empty())
             .take(20)
diff --git a/crates/assists/src/handlers/convert_integer_literal.rs b/crates/assists/src/handlers/convert_integer_literal.rs
new file mode 100644
index 000000000..ea35e833a
--- /dev/null
+++ b/crates/assists/src/handlers/convert_integer_literal.rs
@@ -0,0 +1,701 @@
+use syntax::{ast, AstNode, SmolStr};
+
+use crate::{AssistContext, AssistId, AssistKind, Assists, GroupLabel};
+
+// Assist: convert_integer_literal
+//
+// Converts the base of integer literals to other bases.
+//
+// ```
+// const _: i32 = 10<|>;
+// ```
+// ->
+// ```
+// const _: i32 = 0b1010;
+// ```
+pub(crate) fn convert_integer_literal(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {
+    let literal = ctx.find_node_at_offset::<ast::Literal>()?;
+    let range = literal.syntax().text_range();
+    let group_id = GroupLabel("Convert integer base".into());
+
+    let suffix = match literal.kind() {
+        ast::LiteralKind::IntNumber { suffix } => suffix,
+        _ => return None,
+    };
+    let suffix_len = suffix.as_ref().map(|s| s.len()).unwrap_or(0);
+    let raw_literal_text = literal.syntax().to_string();
+
+    // Gets the literal's text without the type suffix and without underscores.
+    let literal_text = raw_literal_text
+        .chars()
+        .take(raw_literal_text.len() - suffix_len)
+        .filter(|c| *c != '_')
+        .collect::<SmolStr>();
+    let literal_base = IntegerLiteralBase::identify(&literal_text)?;
+
+    for base in IntegerLiteralBase::bases() {
+        if *base == literal_base {
+            continue;
+        }
+
+        let mut converted = literal_base.convert(&literal_text, base);
+
+        let label = if let Some(suffix) = &suffix {
+            format!("Convert {} ({}) to {}", &literal_text, suffix, &converted)
+        } else {
+            format!("Convert {} to {}", &literal_text, &converted)
+        };
+
+        // Appends the type suffix back into the new literal if it exists.
+        if let Some(suffix) = &suffix {
+            converted.push_str(&suffix);
+        }
+
+        acc.add_group(
+            &group_id,
+            AssistId("convert_integer_literal", AssistKind::RefactorInline),
+            label,
+            range,
+            |builder| builder.replace(range, converted),
+        );
+    }
+
+    Some(())
+}
+
+#[derive(Debug, PartialEq, Eq)]
+enum IntegerLiteralBase {
+    Binary,
+    Octal,
+    Decimal,
+    Hexadecimal,
+}
+
+impl IntegerLiteralBase {
+    fn identify(literal_text: &str) -> Option<Self> {
+        // We cannot express a literal in anything other than decimal in under 3 characters, so we return here if possible.
+        if literal_text.len() < 3 && literal_text.chars().all(|c| c.is_digit(10)) {
+            return Some(Self::Decimal);
+        }
+
+        let base = match &literal_text[..2] {
+            "0b" => Self::Binary,
+            "0o" => Self::Octal,
+            "0x" => Self::Hexadecimal,
+            _ => Self::Decimal,
+        };
+
+        // Checks that all characters after the base prefix are all valid digits for that base.
+        if literal_text[base.prefix_len()..].chars().all(|c| c.is_digit(base.base())) {
+            Some(base)
+        } else {
+            None
+        }
+    }
+
+    fn convert(&self, literal_text: &str, to: &IntegerLiteralBase) -> String {
+        let digits = &literal_text[self.prefix_len()..];
+        let value = u128::from_str_radix(digits, self.base()).unwrap();
+
+        match to {
+            Self::Binary => format!("0b{:b}", value),
+            Self::Octal => format!("0o{:o}", value),
+            Self::Decimal => value.to_string(),
+            Self::Hexadecimal => format!("0x{:X}", value),
+        }
+    }
+
+    const fn base(&self) -> u32 {
+        match self {
+            Self::Binary => 2,
+            Self::Octal => 8,
+            Self::Decimal => 10,
+            Self::Hexadecimal => 16,
+        }
+    }
+
+    const fn prefix_len(&self) -> usize {
+        match self {
+            Self::Decimal => 0,
+            _ => 2,
+        }
+    }
+
+    const fn bases() -> &'static [IntegerLiteralBase] {
+        &[
+            IntegerLiteralBase::Binary,
+            IntegerLiteralBase::Octal,
+            IntegerLiteralBase::Decimal,
+            IntegerLiteralBase::Hexadecimal,
+        ]
+    }
+}
+
+#[cfg(test)]
+mod tests {
+
+    use super::*;
+    use crate::tests::{check_assist_by_label, check_assist_target};
+
+    #[test]
+    fn binary_target() {
+        check_assist_target(convert_integer_literal, "const _: i32 = 0b1010<|>;", "0b1010");
+    }
+
+    #[test]
+    fn octal_target() {
+        check_assist_target(convert_integer_literal, "const _: i32 = 0o12<|>;", "0o12");
+    }
+
+    #[test]
+    fn decimal_target() {
+        check_assist_target(convert_integer_literal, "const _: i32 = 10<|>;", "10");
+    }
+
+    #[test]
+    fn hexadecimal_target() {
+        check_assist_target(convert_integer_literal, "const _: i32 = 0xA<|>;", "0xA");
+    }
+
+    #[test]
+    fn binary_target_with_underscores() {
+        check_assist_target(convert_integer_literal, "const _: i32 = 0b10_10<|>;", "0b10_10");
+    }
+
+    #[test]
+    fn octal_target_with_underscores() {
+        check_assist_target(convert_integer_literal, "const _: i32 = 0o1_2<|>;", "0o1_2");
+    }
+
+    #[test]
+    fn decimal_target_with_underscores() {
+        check_assist_target(convert_integer_literal, "const _: i32 = 1_0<|>;", "1_0");
+    }
+
+    #[test]
+    fn hexadecimal_target_with_underscores() {
+        check_assist_target(convert_integer_literal, "const _: i32 = 0x_A<|>;", "0x_A");
+    }
+
+    #[test]
+    fn convert_decimal_integer() {
+        let before = "const _: i32 = 1000<|>;";
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0b1111101000;",
+            "Convert 1000 to 0b1111101000",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0o1750;",
+            "Convert 1000 to 0o1750",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0x3E8;",
+            "Convert 1000 to 0x3E8",
+        );
+    }
+
+    // Decimal numbers under 3 digits have a special case where they return early because we can't fit a
+    // other base's prefix, so we have a separate test for that.
+    #[test]
+    fn convert_small_decimal_integer() {
+        let before = "const _: i32 = 10<|>;";
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0b1010;",
+            "Convert 10 to 0b1010",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0o12;",
+            "Convert 10 to 0o12",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0xA;",
+            "Convert 10 to 0xA",
+        );
+    }
+
+    #[test]
+    fn convert_hexadecimal_integer() {
+        let before = "const _: i32 = 0xFF<|>;";
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0b11111111;",
+            "Convert 0xFF to 0b11111111",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0o377;",
+            "Convert 0xFF to 0o377",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 255;",
+            "Convert 0xFF to 255",
+        );
+    }
+
+    #[test]
+    fn convert_binary_integer() {
+        let before = "const _: i32 = 0b11111111<|>;";
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0o377;",
+            "Convert 0b11111111 to 0o377",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 255;",
+            "Convert 0b11111111 to 255",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0xFF;",
+            "Convert 0b11111111 to 0xFF",
+        );
+    }
+
+    #[test]
+    fn convert_octal_integer() {
+        let before = "const _: i32 = 0o377<|>;";
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0b11111111;",
+            "Convert 0o377 to 0b11111111",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 255;",
+            "Convert 0o377 to 255",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0xFF;",
+            "Convert 0o377 to 0xFF",
+        );
+    }
+
+    #[test]
+    fn convert_decimal_integer_with_underscores() {
+        let before = "const _: i32 = 1_00_0<|>;";
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0b1111101000;",
+            "Convert 1000 to 0b1111101000",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0o1750;",
+            "Convert 1000 to 0o1750",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0x3E8;",
+            "Convert 1000 to 0x3E8",
+        );
+    }
+
+    #[test]
+    fn convert_small_decimal_integer_with_underscores() {
+        let before = "const _: i32 = 1_0<|>;";
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0b1010;",
+            "Convert 10 to 0b1010",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0o12;",
+            "Convert 10 to 0o12",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0xA;",
+            "Convert 10 to 0xA",
+        );
+    }
+
+    #[test]
+    fn convert_hexadecimal_integer_with_underscores() {
+        let before = "const _: i32 = 0x_F_F<|>;";
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0b11111111;",
+            "Convert 0xFF to 0b11111111",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0o377;",
+            "Convert 0xFF to 0o377",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 255;",
+            "Convert 0xFF to 255",
+        );
+    }
+
+    #[test]
+    fn convert_binary_integer_with_underscores() {
+        let before = "const _: i32 = 0b1111_1111<|>;";
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0o377;",
+            "Convert 0b11111111 to 0o377",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 255;",
+            "Convert 0b11111111 to 255",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0xFF;",
+            "Convert 0b11111111 to 0xFF",
+        );
+    }
+
+    #[test]
+    fn convert_octal_integer_with_underscores() {
+        let before = "const _: i32 = 0o3_77<|>;";
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0b11111111;",
+            "Convert 0o377 to 0b11111111",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 255;",
+            "Convert 0o377 to 255",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0xFF;",
+            "Convert 0o377 to 0xFF",
+        );
+    }
+
+    #[test]
+    fn convert_decimal_integer_with_suffix() {
+        let before = "const _: i32 = 1000i32<|>;";
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0b1111101000i32;",
+            "Convert 1000 (i32) to 0b1111101000",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0o1750i32;",
+            "Convert 1000 (i32) to 0o1750",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0x3E8i32;",
+            "Convert 1000 (i32) to 0x3E8",
+        );
+    }
+
+    #[test]
+    fn convert_small_decimal_integer_with_suffix() {
+        let before = "const _: i32 = 10i32<|>;";
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0b1010i32;",
+            "Convert 10 (i32) to 0b1010",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0o12i32;",
+            "Convert 10 (i32) to 0o12",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0xAi32;",
+            "Convert 10 (i32) to 0xA",
+        );
+    }
+
+    #[test]
+    fn convert_hexadecimal_integer_with_suffix() {
+        let before = "const _: i32 = 0xFFi32<|>;";
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0b11111111i32;",
+            "Convert 0xFF (i32) to 0b11111111",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0o377i32;",
+            "Convert 0xFF (i32) to 0o377",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 255i32;",
+            "Convert 0xFF (i32) to 255",
+        );
+    }
+
+    #[test]
+    fn convert_binary_integer_with_suffix() {
+        let before = "const _: i32 = 0b11111111i32<|>;";
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0o377i32;",
+            "Convert 0b11111111 (i32) to 0o377",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 255i32;",
+            "Convert 0b11111111 (i32) to 255",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0xFFi32;",
+            "Convert 0b11111111 (i32) to 0xFF",
+        );
+    }
+
+    #[test]
+    fn convert_octal_integer_with_suffix() {
+        let before = "const _: i32 = 0o377i32<|>;";
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0b11111111i32;",
+            "Convert 0o377 (i32) to 0b11111111",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 255i32;",
+            "Convert 0o377 (i32) to 255",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0xFFi32;",
+            "Convert 0o377 (i32) to 0xFF",
+        );
+    }
+
+    #[test]
+    fn convert_decimal_integer_with_underscores_and_suffix() {
+        let before = "const _: i32 = 1_00_0i32<|>;";
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0b1111101000i32;",
+            "Convert 1000 (i32) to 0b1111101000",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0o1750i32;",
+            "Convert 1000 (i32) to 0o1750",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0x3E8i32;",
+            "Convert 1000 (i32) to 0x3E8",
+        );
+    }
+
+    #[test]
+    fn convert_small_decimal_integer_with_underscores_and_suffix() {
+        let before = "const _: i32 = 1_0i32<|>;";
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0b1010i32;",
+            "Convert 10 (i32) to 0b1010",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0o12i32;",
+            "Convert 10 (i32) to 0o12",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0xAi32;",
+            "Convert 10 (i32) to 0xA",
+        );
+    }
+
+    #[test]
+    fn convert_hexadecimal_integer_with_underscores_and_suffix() {
+        let before = "const _: i32 = 0x_F_Fi32<|>;";
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0b11111111i32;",
+            "Convert 0xFF (i32) to 0b11111111",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0o377i32;",
+            "Convert 0xFF (i32) to 0o377",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 255i32;",
+            "Convert 0xFF (i32) to 255",
+        );
+    }
+
+    #[test]
+    fn convert_binary_integer_with_underscores_and_suffix() {
+        let before = "const _: i32 = 0b1111_1111i32<|>;";
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0o377i32;",
+            "Convert 0b11111111 (i32) to 0o377",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 255i32;",
+            "Convert 0b11111111 (i32) to 255",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0xFFi32;",
+            "Convert 0b11111111 (i32) to 0xFF",
+        );
+    }
+
+    #[test]
+    fn convert_octal_integer_with_underscores_and_suffix() {
+        let before = "const _: i32 = 0o3_77i32<|>;";
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0b11111111i32;",
+            "Convert 0o377 (i32) to 0b11111111",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 255i32;",
+            "Convert 0o377 (i32) to 255",
+        );
+
+        check_assist_by_label(
+            convert_integer_literal,
+            before,
+            "const _: i32 = 0xFFi32;",
+            "Convert 0o377 (i32) to 0xFF",
+        );
+    }
+}
diff --git a/crates/assists/src/handlers/remove_dbg.rs b/crates/assists/src/handlers/remove_dbg.rs
index a8ab2aecc..e10616779 100644
--- a/crates/assists/src/handlers/remove_dbg.rs
+++ b/crates/assists/src/handlers/remove_dbg.rs
@@ -1,6 +1,6 @@
 use syntax::{
     ast::{self, AstNode},
-    SyntaxElement, TextRange, TextSize, T,
+    SyntaxElement, SyntaxKind, TextRange, TextSize, T,
 };
 
 use crate::{AssistContext, AssistId, AssistKind, Assists};
@@ -117,7 +117,10 @@ fn needs_parentheses_around_macro_contents(macro_contents: Vec<SyntaxElement>) -
             }
             symbol_kind => {
                 let symbol_not_in_bracket = unpaired_brackets_in_contents.is_empty();
-                if symbol_not_in_bracket && symbol_kind.is_punct() {
+                if symbol_not_in_bracket
+                    && symbol_kind != SyntaxKind::COLON
+                    && symbol_kind.is_punct()
+                {
                     return true;
                 }
             }
@@ -159,6 +162,8 @@ fn foo(n: usize) {
 }
 ",
         );
+
+        check_assist(remove_dbg, "<|>dbg!(Foo::foo_test()).bar()", "Foo::foo_test().bar()");
     }
 
     #[test]
diff --git a/crates/assists/src/lib.rs b/crates/assists/src/lib.rs
index cbac53e71..a2bec818c 100644
--- a/crates/assists/src/lib.rs
+++ b/crates/assists/src/lib.rs
@@ -128,6 +128,7 @@ mod handlers {
     mod auto_import;
     mod change_return_type_to_result;
     mod change_visibility;
+    mod convert_integer_literal;
     mod early_return;
     mod expand_glob_import;
     mod extract_struct_from_enum_variant;
@@ -172,6 +173,7 @@ mod handlers {
             auto_import::auto_import,
             change_return_type_to_result::change_return_type_to_result,
             change_visibility::change_visibility,
+            convert_integer_literal::convert_integer_literal,
             early_return::convert_to_guarded_return,
             expand_glob_import::expand_glob_import,
             extract_struct_from_enum_variant::extract_struct_from_enum_variant,
diff --git a/crates/assists/src/tests.rs b/crates/assists/src/tests.rs
index ba1fb543b..2b687decf 100644
--- a/crates/assists/src/tests.rs
+++ b/crates/assists/src/tests.rs
@@ -15,18 +15,30 @@ pub(crate) fn with_single_file(text: &str) -> (RootDatabase, FileId) {
 
 pub(crate) fn check_assist(assist: Handler, ra_fixture_before: &str, ra_fixture_after: &str) {
     let ra_fixture_after = trim_indent(ra_fixture_after);
-    check(assist, ra_fixture_before, ExpectedResult::After(&ra_fixture_after));
+    check(assist, ra_fixture_before, ExpectedResult::After(&ra_fixture_after), None);
+}
+
+// There is no way to choose what assist within a group you want to test against,
+// so this is here to allow you choose.
+pub(crate) fn check_assist_by_label(
+    assist: Handler,
+    ra_fixture_before: &str,
+    ra_fixture_after: &str,
+    label: &str,
+) {
+    let ra_fixture_after = trim_indent(ra_fixture_after);
+    check(assist, ra_fixture_before, ExpectedResult::After(&ra_fixture_after), Some(label));
 }
 
 // FIXME: instead of having a separate function here, maybe use
 // `extract_ranges` and mark the target as `<target> </target>` in the
 // fixture?
 pub(crate) fn check_assist_target(assist: Handler, ra_fixture: &str, target: &str) {
-    check(assist, ra_fixture, ExpectedResult::Target(target));
+    check(assist, ra_fixture, ExpectedResult::Target(target), None);
 }
 
 pub(crate) fn check_assist_not_applicable(assist: Handler, ra_fixture: &str) {
-    check(assist, ra_fixture, ExpectedResult::NotApplicable);
+    check(assist, ra_fixture, ExpectedResult::NotApplicable, None);
 }
 
 fn check_doc_test(assist_id: &str, before: &str, after: &str) {
@@ -65,7 +77,7 @@ enum ExpectedResult<'a> {
     Target(&'a str),
 }
 
-fn check(handler: Handler, before: &str, expected: ExpectedResult) {
+fn check(handler: Handler, before: &str, expected: ExpectedResult, assist_label: Option<&str>) {
     let (db, file_with_caret_id, range_or_offset) = RootDatabase::with_range_or_offset(before);
     let text_without_caret = db.file_text(file_with_caret_id).to_string();
 
@@ -77,7 +89,12 @@ fn check(handler: Handler, before: &str, expected: ExpectedResult) {
     let mut acc = Assists::new_resolved(&ctx);
     handler(&mut acc, &ctx);
     let mut res = acc.finish_resolved();
-    let assist = res.pop();
+
+    let assist = match assist_label {
+        Some(label) => res.into_iter().find(|resolved| resolved.assist.label == label),
+        None => res.pop(),
+    };
+
     match (assist, expected) {
         (Some(assist), ExpectedResult::After(after)) => {
             let mut source_change = assist.source_change;
diff --git a/crates/assists/src/tests/generated.rs b/crates/assists/src/tests/generated.rs
index 27d15adb0..7f6e98a54 100644
--- a/crates/assists/src/tests/generated.rs
+++ b/crates/assists/src/tests/generated.rs
@@ -204,6 +204,19 @@ pub(crate) fn frobnicate() {}
 }
 
 #[test]
+fn doctest_convert_integer_literal() {
+    check_doc_test(
+        "convert_integer_literal",
+        r#####"
+const _: i32 = 10<|>;
+"#####,
+        r#####"
+const _: i32 = 0b1010;
+"#####,
+    )
+}
+
+#[test]
 fn doctest_convert_to_guarded_return() {
     check_doc_test(
         "convert_to_guarded_return",
diff --git a/crates/hir/src/code_model.rs b/crates/hir/src/code_model.rs
index a19b0fad7..c75d46bff 100644
--- a/crates/hir/src/code_model.rs
+++ b/crates/hir/src/code_model.rs
@@ -4,6 +4,7 @@ use std::{iter, sync::Arc};
 use arrayvec::ArrayVec;
 use base_db::{CrateId, CrateName, Edition, FileId};
 use either::Either;
+use hir_def::find_path::PrefixKind;
 use hir_def::{
     adt::ReprKind,
     adt::StructKind,
@@ -390,8 +391,9 @@ impl Module {
         self,
         db: &dyn DefDatabase,
         item: impl Into<ItemInNs>,
+        prefix_kind: PrefixKind,
     ) -> Option<ModPath> {
-        hir_def::find_path::find_path_prefixed(db, item.into(), self.into())
+        hir_def::find_path::find_path_prefixed(db, item.into(), self.into(), prefix_kind)
     }
 }
 
diff --git a/crates/hir/src/lib.rs b/crates/hir/src/lib.rs
index b9d9c7e25..87084fa13 100644
--- a/crates/hir/src/lib.rs
+++ b/crates/hir/src/lib.rs
@@ -48,6 +48,7 @@ pub use hir_def::{
     body::scope::ExprScopes,
     builtin_type::BuiltinType,
     docs::Documentation,
+    find_path::PrefixKind,
     item_scope::ItemInNs,
     nameres::ModuleSource,
     path::ModPath,
diff --git a/crates/hir_def/src/find_path.rs b/crates/hir_def/src/find_path.rs
index baf374144..9106ed45f 100644
--- a/crates/hir_def/src/find_path.rs
+++ b/crates/hir_def/src/find_path.rs
@@ -19,12 +19,17 @@ use crate::{
 /// *from where* you're referring to the item, hence the `from` parameter.
 pub fn find_path(db: &dyn DefDatabase, item: ItemInNs, from: ModuleId) -> Option<ModPath> {
     let _p = profile::span("find_path");
-    find_path_inner(db, item, from, MAX_PATH_LEN, Prefixed::Not)
+    find_path_inner(db, item, from, MAX_PATH_LEN, None)
 }
 
-pub fn find_path_prefixed(db: &dyn DefDatabase, item: ItemInNs, from: ModuleId) -> Option<ModPath> {
-    let _p = profile::span("find_path_absolute");
-    find_path_inner(db, item, from, MAX_PATH_LEN, Prefixed::Plain)
+pub fn find_path_prefixed(
+    db: &dyn DefDatabase,
+    item: ItemInNs,
+    from: ModuleId,
+    prefix_kind: PrefixKind,
+) -> Option<ModPath> {
+    let _p = profile::span("find_path_prefixed");
+    find_path_inner(db, item, from, MAX_PATH_LEN, Some(prefix_kind))
 }
 
 const MAX_PATH_LEN: usize = 15;
@@ -42,58 +47,52 @@ impl ModPath {
     }
 }
 
-fn check_crate_self_super(
-    def_map: &CrateDefMap,
-    item: ItemInNs,
-    from: ModuleId,
-) -> Option<ModPath> {
-    // - if the item is the crate root, return `crate`
-    if item
-        == ItemInNs::Types(ModuleDefId::ModuleId(ModuleId {
-            krate: from.krate,
-            local_id: def_map.root,
-        }))
-    {
-        Some(ModPath::from_segments(PathKind::Crate, Vec::new()))
-    } else if item == ItemInNs::Types(from.into()) {
+fn check_self_super(def_map: &CrateDefMap, item: ItemInNs, from: ModuleId) -> Option<ModPath> {
+    if item == ItemInNs::Types(from.into()) {
         // - if the item is the module we're in, use `self`
         Some(ModPath::from_segments(PathKind::Super(0), Vec::new()))
-    } else {
-        if let Some(parent_id) = def_map.modules[from.local_id].parent {
-            // - if the item is the parent module, use `super` (this is not used recursively, since `super::super` is ugly)
-            if item
-                == ItemInNs::Types(ModuleDefId::ModuleId(ModuleId {
-                    krate: from.krate,
-                    local_id: parent_id,
-                }))
-            {
-                return Some(ModPath::from_segments(PathKind::Super(1), Vec::new()));
-            }
+    } else if let Some(parent_id) = def_map.modules[from.local_id].parent {
+        // - if the item is the parent module, use `super` (this is not used recursively, since `super::super` is ugly)
+        if item
+            == ItemInNs::Types(ModuleDefId::ModuleId(ModuleId {
+                krate: from.krate,
+                local_id: parent_id,
+            }))
+        {
+            Some(ModPath::from_segments(PathKind::Super(1), Vec::new()))
+        } else {
+            None
         }
+    } else {
         None
     }
 }
 
-#[derive(Copy, Clone, PartialEq, Eq)]
-pub enum Prefixed {
-    Not,
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub enum PrefixKind {
+    /// Causes paths to always start with either `self`, `super`, `crate` or a crate-name.
+    /// This is the same as plain, just that paths will start with `self` iprepended f the path
+    /// starts with an identifier that is not a crate.
     BySelf,
+    /// Causes paths to ignore imports in the local module.
     Plain,
+    /// Causes paths to start with `crate` where applicable, effectively forcing paths to be absolute.
+    ByCrate,
 }
 
-impl Prefixed {
+impl PrefixKind {
     #[inline]
-    fn prefix(self) -> Option<PathKind> {
+    fn prefix(self) -> PathKind {
         match self {
-            Prefixed::Not => None,
-            Prefixed::BySelf => Some(PathKind::Super(0)),
-            Prefixed::Plain => Some(PathKind::Plain),
+            PrefixKind::BySelf => PathKind::Super(0),
+            PrefixKind::Plain => PathKind::Plain,
+            PrefixKind::ByCrate => PathKind::Crate,
         }
     }
 
     #[inline]
-    fn prefixed(self) -> bool {
-        self != Prefixed::Not
+    fn is_absolute(&self) -> bool {
+        self == &PrefixKind::ByCrate
     }
 }
 
@@ -102,7 +101,7 @@ fn find_path_inner(
     item: ItemInNs,
     from: ModuleId,
     max_len: usize,
-    prefixed: Prefixed,
+    prefixed: Option<PrefixKind>,
 ) -> Option<ModPath> {
     if max_len == 0 {
         return None;
@@ -115,13 +114,25 @@ fn find_path_inner(
     let from_scope: &crate::item_scope::ItemScope = &def_map.modules[from.local_id].scope;
     let scope_name =
         if let Some((name, _)) = from_scope.name_of(item) { Some(name.clone()) } else { None };
-    if !prefixed.prefixed() && scope_name.is_some() {
+    if prefixed.is_none() && scope_name.is_some() {
         return scope_name
             .map(|scope_name| ModPath::from_segments(PathKind::Plain, vec![scope_name]));
     }
 
-    if let modpath @ Some(_) = check_crate_self_super(&def_map, item, from) {
-        return modpath;
+    // - if the item is the crate root, return `crate`
+    if item
+        == ItemInNs::Types(ModuleDefId::ModuleId(ModuleId {
+            krate: from.krate,
+            local_id: def_map.root,
+        }))
+    {
+        return Some(ModPath::from_segments(PathKind::Crate, Vec::new()));
+    }
+
+    if prefixed.filter(PrefixKind::is_absolute).is_none() {
+        if let modpath @ Some(_) = check_self_super(&def_map, item, from) {
+            return modpath;
+        }
     }
 
     // - if the item is the crate root of a dependency crate, return the name from the extern prelude
@@ -226,7 +237,7 @@ fn find_path_inner(
         }
     }
 
-    if let Some(prefix) = prefixed.prefix() {
+    if let Some(prefix) = prefixed.map(PrefixKind::prefix) {
         best_path.or_else(|| {
             scope_name.map(|scope_name| ModPath::from_segments(prefix, vec![scope_name]))
         })
@@ -355,7 +366,7 @@ mod tests {
     /// `code` needs to contain a cursor marker; checks that `find_path` for the
     /// item the `path` refers to returns that same path when called from the
     /// module the cursor is in.
-    fn check_found_path_(ra_fixture: &str, path: &str, absolute: bool) {
+    fn check_found_path_(ra_fixture: &str, path: &str, prefix_kind: Option<PrefixKind>) {
         let (db, pos) = TestDB::with_position(ra_fixture);
         let module = db.module_for_file(pos.file_id);
         let parsed_path_file = syntax::SourceFile::parse(&format!("use {};", path));
@@ -375,20 +386,22 @@ mod tests {
             .take_types()
             .unwrap();
 
-        let found_path = if absolute { find_path_prefixed } else { find_path }(
-            &db,
-            ItemInNs::Types(resolved),
-            module,
-        );
-        assert_eq!(found_path, Some(mod_path), "absolute {}", absolute);
+        let found_path =
+            find_path_inner(&db, ItemInNs::Types(resolved), module, MAX_PATH_LEN, prefix_kind);
+        assert_eq!(found_path, Some(mod_path), "{:?}", prefix_kind);
     }
 
-    fn check_found_path(ra_fixture: &str, path: &str) {
-        check_found_path_(ra_fixture, path, false);
-    }
-
-    fn check_found_path_abs(ra_fixture: &str, path: &str) {
-        check_found_path_(ra_fixture, path, true);
+    fn check_found_path(
+        ra_fixture: &str,
+        unprefixed: &str,
+        prefixed: &str,
+        absolute: &str,
+        self_prefixed: &str,
+    ) {
+        check_found_path_(ra_fixture, unprefixed, None);
+        check_found_path_(ra_fixture, prefixed, Some(PrefixKind::Plain));
+        check_found_path_(ra_fixture, absolute, Some(PrefixKind::ByCrate));
+        check_found_path_(ra_fixture, self_prefixed, Some(PrefixKind::BySelf));
     }
 
     #[test]
@@ -398,8 +411,7 @@ mod tests {
             struct S;
             <|>
         "#;
-        check_found_path(code, "S");
-        check_found_path_abs(code, "S");
+        check_found_path(code, "S", "S", "crate::S", "self::S");
     }
 
     #[test]
@@ -409,8 +421,7 @@ mod tests {
             enum E { A }
             <|>
         "#;
-        check_found_path(code, "E::A");
-        check_found_path_abs(code, "E::A");
+        check_found_path(code, "E::A", "E::A", "E::A", "E::A");
     }
 
     #[test]
@@ -422,8 +433,7 @@ mod tests {
             }
             <|>
         "#;
-        check_found_path(code, "foo::S");
-        check_found_path_abs(code, "foo::S");
+        check_found_path(code, "foo::S", "foo::S", "crate::foo::S", "self::foo::S");
     }
 
     #[test]
@@ -437,8 +447,7 @@ mod tests {
             //- /foo/bar.rs
             <|>
         "#;
-        check_found_path(code, "super::S");
-        check_found_path_abs(code, "super::S");
+        check_found_path(code, "super::S", "super::S", "crate::foo::S", "super::S");
     }
 
     #[test]
@@ -449,8 +458,7 @@ mod tests {
             //- /foo.rs
             <|>
         "#;
-        check_found_path(code, "self");
-        check_found_path_abs(code, "self");
+        check_found_path(code, "self", "self", "crate::foo", "self");
     }
 
     #[test]
@@ -461,8 +469,7 @@ mod tests {
             //- /foo.rs
             <|>
         "#;
-        check_found_path(code, "crate");
-        check_found_path_abs(code, "crate");
+        check_found_path(code, "crate", "crate", "crate", "crate");
     }
 
     #[test]
@@ -474,8 +481,7 @@ mod tests {
             //- /foo.rs
             <|>
         "#;
-        check_found_path(code, "crate::S");
-        check_found_path_abs(code, "crate::S");
+        check_found_path(code, "crate::S", "crate::S", "crate::S", "crate::S");
     }
 
     #[test]
@@ -486,8 +492,7 @@ mod tests {
             //- /std.rs crate:std
             pub struct S;
         "#;
-        check_found_path(code, "std::S");
-        check_found_path_abs(code, "std::S");
+        check_found_path(code, "std::S", "std::S", "std::S", "std::S");
     }
 
     #[test]
@@ -499,8 +504,13 @@ mod tests {
             //- /std.rs crate:std
             pub struct S;
         "#;
-        check_found_path(code, "std_renamed::S");
-        check_found_path_abs(code, "std_renamed::S");
+        check_found_path(
+            code,
+            "std_renamed::S",
+            "std_renamed::S",
+            "std_renamed::S",
+            "std_renamed::S",
+        );
     }
 
     #[test]
@@ -520,8 +530,13 @@ mod tests {
                 }
             }
         "#;
-        check_found_path(code, "ast::ModuleItem");
-        check_found_path_abs(code, "syntax::ast::ModuleItem");
+        check_found_path(
+            code,
+            "ast::ModuleItem",
+            "syntax::ast::ModuleItem",
+            "syntax::ast::ModuleItem",
+            "syntax::ast::ModuleItem",
+        );
 
         let code = r#"
             //- /main.rs crate:main deps:syntax
@@ -535,8 +550,13 @@ mod tests {
                 }
             }
         "#;
-        check_found_path(code, "syntax::ast::ModuleItem");
-        check_found_path_abs(code, "syntax::ast::ModuleItem");
+        check_found_path(
+            code,
+            "syntax::ast::ModuleItem",
+            "syntax::ast::ModuleItem",
+            "syntax::ast::ModuleItem",
+            "syntax::ast::ModuleItem",
+        );
     }
 
     #[test]
@@ -549,8 +569,7 @@ mod tests {
             }
             <|>
         "#;
-        check_found_path(code, "bar::S");
-        check_found_path_abs(code, "bar::S");
+        check_found_path(code, "bar::S", "bar::S", "crate::bar::S", "self::bar::S");
     }
 
     #[test]
@@ -563,8 +582,7 @@ mod tests {
             }
             <|>
         "#;
-        check_found_path(code, "bar::U");
-        check_found_path_abs(code, "bar::U");
+        check_found_path(code, "bar::U", "bar::U", "crate::bar::U", "self::bar::U");
     }
 
     #[test]
@@ -577,8 +595,7 @@ mod tests {
             //- /core.rs crate:core
             pub struct S;
         "#;
-        check_found_path(code, "std::S");
-        check_found_path_abs(code, "std::S");
+        check_found_path(code, "std::S", "std::S", "std::S", "std::S");
     }
 
     #[test]
@@ -591,8 +608,7 @@ mod tests {
             #[prelude_import]
             pub use prelude::*;
         "#;
-        check_found_path(code, "S");
-        check_found_path_abs(code, "S");
+        check_found_path(code, "S", "S", "S", "S");
     }
 
     #[test]
@@ -608,10 +624,8 @@ mod tests {
             #[prelude_import]
             pub use prelude::*;
         "#;
-        check_found_path(code, "None");
-        check_found_path(code, "Some");
-        check_found_path_abs(code, "None");
-        check_found_path_abs(code, "Some");
+        check_found_path(code, "None", "None", "None", "None");
+        check_found_path(code, "Some", "Some", "Some", "Some");
     }
 
     #[test]
@@ -627,8 +641,7 @@ mod tests {
             //- /baz.rs
             pub use crate::foo::bar::S;
         "#;
-        check_found_path(code, "baz::S");
-        check_found_path_abs(code, "baz::S");
+        check_found_path(code, "baz::S", "baz::S", "crate::baz::S", "self::baz::S");
     }
 
     #[test]
@@ -642,8 +655,7 @@ mod tests {
             <|>
         "#;
         // crate::S would be shorter, but using private imports seems wrong
-        check_found_path(code, "crate::bar::S");
-        check_found_path_abs(code, "crate::bar::S");
+        check_found_path(code, "crate::bar::S", "crate::bar::S", "crate::bar::S", "crate::bar::S");
     }
 
     #[test]
@@ -661,8 +673,7 @@ mod tests {
             //- /baz.rs
             pub use super::foo;
         "#;
-        check_found_path(code, "crate::foo::S");
-        check_found_path_abs(code, "crate::foo::S");
+        check_found_path(code, "crate::foo::S", "crate::foo::S", "crate::foo::S", "crate::foo::S");
     }
 
     #[test]
@@ -682,8 +693,13 @@ mod tests {
             pub struct Arc;
         }
         "#;
-        check_found_path(code, "std::sync::Arc");
-        check_found_path_abs(code, "std::sync::Arc");
+        check_found_path(
+            code,
+            "std::sync::Arc",
+            "std::sync::Arc",
+            "std::sync::Arc",
+            "std::sync::Arc",
+        );
     }
 
     #[test]
@@ -707,8 +723,13 @@ mod tests {
             pub struct Error;
         }
         "#;
-        check_found_path(code, "core::fmt::Error");
-        check_found_path_abs(code, "core::fmt::Error");
+        check_found_path(
+            code,
+            "core::fmt::Error",
+            "core::fmt::Error",
+            "core::fmt::Error",
+            "core::fmt::Error",
+        );
     }
 
     #[test]
@@ -731,8 +752,13 @@ mod tests {
             pub struct Arc;
         }
         "#;
-        check_found_path(code, "alloc::sync::Arc");
-        check_found_path_abs(code, "alloc::sync::Arc");
+        check_found_path(
+            code,
+            "alloc::sync::Arc",
+            "alloc::sync::Arc",
+            "alloc::sync::Arc",
+            "alloc::sync::Arc",
+        );
     }
 
     #[test]
@@ -749,8 +775,13 @@ mod tests {
         //- /zzz.rs crate:megaalloc
         pub struct Arc;
         "#;
-        check_found_path(code, "megaalloc::Arc");
-        check_found_path_abs(code, "megaalloc::Arc");
+        check_found_path(
+            code,
+            "megaalloc::Arc",
+            "megaalloc::Arc",
+            "megaalloc::Arc",
+            "megaalloc::Arc",
+        );
     }
 
     #[test]
@@ -763,9 +794,7 @@ mod tests {
             pub use u8;
         }
         "#;
-        check_found_path(code, "u8");
-        check_found_path(code, "u16");
-        check_found_path_abs(code, "u8");
-        check_found_path_abs(code, "u16");
+        check_found_path(code, "u8", "u8", "u8", "u8");
+        check_found_path(code, "u16", "u16", "u16", "u16");
     }
 }
diff --git a/crates/hir_def/src/nameres/collector.rs b/crates/hir_def/src/nameres/collector.rs
index 100e25ffc..c8cd04264 100644
--- a/crates/hir_def/src/nameres/collector.rs
+++ b/crates/hir_def/src/nameres/collector.rs
@@ -1229,9 +1229,10 @@ impl ModCollector<'_, '_> {
         } else {
             let derive = attrs.by_key("proc_macro_derive");
             if let Some(arg) = derive.tt_values().next() {
-                if let [TokenTree::Leaf(Leaf::Ident(trait_name))] = &*arg.token_trees {
+                if let [TokenTree::Leaf(Leaf::Ident(trait_name)), ..] = &*arg.token_trees {
                     trait_name.as_name()
                 } else {
+                    log::trace!("malformed `#[proc_macro_derive]`: {}", arg);
                     return;
                 }
             } else {
diff --git a/crates/hir_def/src/nameres/tests/macros.rs b/crates/hir_def/src/nameres/tests/macros.rs
index 0851c3b7d..305fca0f9 100644
--- a/crates/hir_def/src/nameres/tests/macros.rs
+++ b/crates/hir_def/src/nameres/tests/macros.rs
@@ -688,13 +688,20 @@ fn resolves_proc_macros() {
         pub fn derive_macro(_item: TokenStream) -> TokenStream {
             TokenStream
         }
+
+        #[proc_macro_derive(AnotherTrait, attributes(helper_attr))]
+        pub fn derive_macro_2(_item: TokenStream) -> TokenStream {
+            TokenStream
+        }
         ",
         expect![[r#"
             crate
+            AnotherTrait: m
             DummyTrait: m
             TokenStream: t v
             attribute_macro: v m
             derive_macro: v
+            derive_macro_2: v
             function_like_macro: v m
         "#]],
     );
diff --git a/crates/ide/src/completion/complete_postfix/format_like.rs b/crates/ide/src/completion/complete_postfix/format_like.rs
index 0287fc803..81c33bf3a 100644
--- a/crates/ide/src/completion/complete_postfix/format_like.rs
+++ b/crates/ide/src/completion/complete_postfix/format_like.rs
@@ -1,4 +1,4 @@
-// Feature: Postfix completion for `format`-like strings.
+// Feature: Format String Completion.
 //
 // `"Result {result} is {2 + 2}"` is expanded to the `"Result {} is {}", result, 2 + 2`.
 //
diff --git a/crates/ide/src/completion/complete_unqualified_path.rs b/crates/ide/src/completion/complete_unqualified_path.rs
index 2010d9a2f..8b6757195 100644
--- a/crates/ide/src/completion/complete_unqualified_path.rs
+++ b/crates/ide/src/completion/complete_unqualified_path.rs
@@ -267,6 +267,26 @@ fn quux() { <|> }
         );
     }
 
+    /// Regression test for issue #6091.
+    #[test]
+    fn correctly_completes_module_items_prefixed_with_underscore() {
+        check_edit(
+            "_alpha",
+            r#"
+fn main() {
+    _<|>
+}
+fn _alpha() {}
+"#,
+            r#"
+fn main() {
+    _alpha()$0
+}
+fn _alpha() {}
+"#,
+        )
+    }
+
     #[test]
     fn completes_extern_prelude() {
         check(
diff --git a/crates/ide/src/completion/completion_context.rs b/crates/ide/src/completion/completion_context.rs
index 101be8eb5..8dea8a4bf 100644
--- a/crates/ide/src/completion/completion_context.rs
+++ b/crates/ide/src/completion/completion_context.rs
@@ -221,10 +221,11 @@ impl<'a> CompletionContext<'a> {
         Some(ctx)
     }
 
-    // The range of the identifier that is being completed.
+    /// The range of the identifier that is being completed.
     pub(crate) fn source_range(&self) -> TextRange {
         // check kind of macro-expanded token, but use range of original token
-        if self.token.kind() == IDENT || self.token.kind().is_keyword() {
+        let kind = self.token.kind();
+        if kind == IDENT || kind == UNDERSCORE || kind.is_keyword() {
             mark::hit!(completes_if_prefix_is_keyword);
             self.original_token.text_range()
         } else {
diff --git a/crates/ide/src/inlay_hints.rs b/crates/ide/src/inlay_hints.rs
index 0afe5f8fd..1d7e8de56 100644
--- a/crates/ide/src/inlay_hints.rs
+++ b/crates/ide/src/inlay_hints.rs
@@ -189,7 +189,7 @@ fn get_bind_pat_hints(
 
     let ty = sema.type_of_pat(&pat.clone().into())?;
 
-    if should_not_display_type_hint(sema.db, &pat, &ty) {
+    if should_not_display_type_hint(sema, &pat, &ty) {
         return None;
     }
 
@@ -215,10 +215,12 @@ fn pat_is_enum_variant(db: &RootDatabase, bind_pat: &ast::IdentPat, pat_ty: &Typ
 }
 
 fn should_not_display_type_hint(
-    db: &RootDatabase,
+    sema: &Semantics<RootDatabase>,
     bind_pat: &ast::IdentPat,
     pat_ty: &Type,
 ) -> bool {
+    let db = sema.db;
+
     if pat_ty.is_unknown() {
         return true;
     }
@@ -249,6 +251,15 @@ fn should_not_display_type_hint(
                     return it.condition().and_then(|condition| condition.pat()).is_some()
                         && pat_is_enum_variant(db, bind_pat, pat_ty);
                 },
+                ast::ForExpr(it) => {
+                    // We *should* display hint only if user provided "in {expr}" and we know the type of expr (and it's not unit).
+                    // Type of expr should be iterable.
+                    return it.in_token().is_none() ||
+                        it.iterable()
+                            .and_then(|iterable_expr|sema.type_of_expr(&iterable_expr))
+                            .map(|iterable_ty| iterable_ty.is_unknown() || iterable_ty.is_unit())
+                            .unwrap_or(true)
+                },
                 _ => (),
             }
         }
@@ -496,19 +507,6 @@ fn main() {
     }
 
     #[test]
-    fn for_expression() {
-        check(
-            r#"
-fn main() {
-    let mut start = 0;
-      //^^^^^^^^^ i32
-    for increment in 0..2 { start += increment; }
-      //^^^^^^^^^ i32
-}"#,
-        );
-    }
-
-    #[test]
     fn if_expr() {
         check(
             r#"
@@ -924,4 +922,108 @@ fn main() {
             "#]],
         );
     }
+
+    #[test]
+    fn incomplete_for_no_hint() {
+        check(
+            r#"
+fn main() {
+    let data = &[1i32, 2, 3];
+      //^^^^ &[i32; _]
+    for i
+}"#,
+        );
+        check(
+            r#"
+//- /main.rs crate:main deps:core
+pub struct Vec<T> {}
+
+impl<T> Vec<T> {
+    pub fn new() -> Self { Vec {} }
+    pub fn push(&mut self, t: T) {}
+}
+
+impl<T> IntoIterator for Vec<T> {
+    type Item=T;
+}
+
+fn main() {
+    let mut data = Vec::new();
+      //^^^^^^^^ Vec<&str>
+    data.push("foo");
+    for i in
+
+    println!("Unit expr");
+}
+
+//- /core.rs crate:core
+#[prelude_import] use iter::*;
+mod iter {
+    trait IntoIterator {
+        type Item;
+    }
+}
+//- /alloc.rs crate:alloc deps:core
+mod collections {
+    struct Vec<T> {}
+    impl<T> Vec<T> {
+        fn new() -> Self { Vec {} }
+        fn push(&mut self, t: T) { }
+    }
+    impl<T> IntoIterator for Vec<T> {
+        type Item=T;
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn complete_for_hint() {
+        check(
+            r#"
+//- /main.rs crate:main deps:core
+pub struct Vec<T> {}
+
+impl<T> Vec<T> {
+    pub fn new() -> Self { Vec {} }
+    pub fn push(&mut self, t: T) {}
+}
+
+impl<T> IntoIterator for Vec<T> {
+    type Item=T;
+}
+
+fn main() {
+    let mut data = Vec::new();
+      //^^^^^^^^ Vec<&str>
+    data.push("foo");
+    for i in data {
+      //^ &str
+      let z = i;
+        //^ &str
+    }
+}
+
+//- /core.rs crate:core
+#[prelude_import] use iter::*;
+mod iter {
+    trait IntoIterator {
+        type Item;
+    }
+}
+//- /alloc.rs crate:alloc deps:core
+mod collections {
+    struct Vec<T> {}
+    impl<T> Vec<T> {
+        fn new() -> Self { Vec {} }
+        fn push(&mut self, t: T) { }
+    }
+    impl<T> IntoIterator for Vec<T> {
+        type Item=T;
+    }
+}
+"#,
+        );
+    }
 }
diff --git a/crates/rust-analyzer/src/config.rs b/crates/rust-analyzer/src/config.rs
index 0ab4c37bf..dcbc11c14 100644
--- a/crates/rust-analyzer/src/config.rs
+++ b/crates/rust-analyzer/src/config.rs
@@ -10,6 +10,7 @@
 use std::{ffi::OsString, path::PathBuf};
 
 use flycheck::FlycheckConfig;
+use hir::PrefixKind;
 use ide::{
     AssistConfig, CompletionConfig, DiagnosticsConfig, HoverConfig, InlayHintsConfig,
     MergeBehaviour,
@@ -289,6 +290,11 @@ impl Config {
             MergeBehaviourDef::Full => Some(MergeBehaviour::Full),
             MergeBehaviourDef::Last => Some(MergeBehaviour::Last),
         };
+        self.assist.insert_use.prefix_kind = match data.assist_importPrefix {
+            ImportPrefixDef::Plain => PrefixKind::Plain,
+            ImportPrefixDef::ByCrate => PrefixKind::ByCrate,
+            ImportPrefixDef::BySelf => PrefixKind::BySelf,
+        };
 
         self.call_info_full = data.callInfo_full;
 
@@ -403,13 +409,21 @@ enum ManifestOrProjectJson {
 }
 
 #[derive(Deserialize)]
-#[serde(rename_all = "lowercase")]
+#[serde(rename_all = "snake_case")]
 enum MergeBehaviourDef {
     None,
     Full,
     Last,
 }
 
+#[derive(Deserialize)]
+#[serde(rename_all = "snake_case")]
+enum ImportPrefixDef {
+    Plain,
+    BySelf,
+    ByCrate,
+}
+
 macro_rules! config_data {
     (struct $name:ident { $($field:ident: $ty:ty = $default:expr,)*}) => {
         #[allow(non_snake_case)]
@@ -434,6 +448,7 @@ macro_rules! config_data {
 config_data! {
     struct ConfigData {
         assist_importMergeBehaviour: MergeBehaviourDef = MergeBehaviourDef::None,
+        assist_importPrefix: ImportPrefixDef           = ImportPrefixDef::Plain,
 
         callInfo_full: bool = true,