4 files changed, 105 insertions, 3 deletions

diff --git a/crates/assists/src/handlers/invert_if.rs b/crates/assists/src/handlers/invert_if.rs
index f0e047538..294256297 100644
--- a/crates/assists/src/handlers/invert_if.rs
+++ b/crates/assists/src/handlers/invert_if.rs
@@ -106,4 +106,22 @@ mod tests {
             "fn f() { i<|>f let Some(_) = Some(1) { 1 } else { 0 } }",
         )
     }
+
+    #[test]
+    fn invert_if_option_case() {
+        check_assist(
+            invert_if,
+            "fn f() { if<|> doc_style.is_some() { Class::DocComment } else { Class::Comment } }",
+            "fn f() { if doc_style.is_none() { Class::Comment } else { Class::DocComment } }",
+        )
+    }
+
+    #[test]
+    fn invert_if_result_case() {
+        check_assist(
+            invert_if,
+            "fn f() { i<|>f doc_style.is_err() { Class::Err } else { Class::Ok } }",
+            "fn f() { if doc_style.is_ok() { Class::Ok } else { Class::Err } }",
+        )
+    }
 }
diff --git a/crates/assists/src/utils.rs b/crates/assists/src/utils.rs
index d071d6502..e15c982e7 100644
--- a/crates/assists/src/utils.rs
+++ b/crates/assists/src/utils.rs
@@ -11,7 +11,7 @@ use syntax::{
     ast::{self, make, NameOwner},
     AstNode, Direction,
     SyntaxKind::*,
-    SyntaxNode, TextSize, T,
+    SyntaxNode, SyntaxText, TextSize, T,
 };
 
 use crate::assist_config::SnippetCap;
@@ -179,6 +179,25 @@ fn invert_special_case(expr: &ast::Expr) -> Option<ast::Expr> {
             ast::BinOp::EqualityTest => bin.replace_op(T![!=]).map(|it| it.into()),
             _ => None,
         },
+        ast::Expr::MethodCallExpr(mce) => {
+            const IS_SOME_TEXT: &str = "is_some";
+            const IS_NONE_TEXT: &str = "is_none";
+            const IS_OK_TEXT: &str = "is_ok";
+            const IS_ERR_TEXT: &str = "is_err";
+
+            let name = mce.name_ref()?;
+            let name_text = name.text();
+
+            let caller = || -> Option<SyntaxText> { Some(mce.receiver()?.syntax().text()) };
+
+            match name_text {
+                x if x == IS_SOME_TEXT => make::expr_method_call(IS_NONE_TEXT, caller),
+                x if x == IS_NONE_TEXT => make::expr_method_call(IS_SOME_TEXT, caller),
+                x if x == IS_OK_TEXT => make::expr_method_call(IS_ERR_TEXT, caller),
+                x if x == IS_ERR_TEXT => make::expr_method_call(IS_OK_TEXT, caller),
+                _ => None,
+            }
+        }
         ast::Expr::PrefixExpr(pe) if pe.op_kind()? == ast::PrefixOp::Not => pe.expr(),
         // FIXME:
         // ast::Expr::Literal(true | false )
diff --git a/crates/syntax/src/ast/make.rs b/crates/syntax/src/ast/make.rs
index d20c085aa..7958721e2 100644
--- a/crates/syntax/src/ast/make.rs
+++ b/crates/syntax/src/ast/make.rs
@@ -7,7 +7,7 @@
 use itertools::Itertools;
 use stdx::format_to;
 
-use crate::{ast, AstNode, SourceFile, SyntaxKind, SyntaxNode, SyntaxToken};
+use crate::{ast, AstNode, SourceFile, SyntaxKind, SyntaxNode, SyntaxText, SyntaxToken};
 
 pub fn name(text: &str) -> ast::Name {
     ast_from_text(&format!("mod {};", text))
@@ -137,6 +137,12 @@ pub fn expr_prefix(op: SyntaxKind, expr: ast::Expr) -> ast::Expr {
 pub fn expr_call(f: ast::Expr, arg_list: ast::ArgList) -> ast::Expr {
     expr_from_text(&format!("{}{}", f, arg_list))
 }
+pub fn expr_method_call<F>(text: &str, caller: F) -> Option<ast::Expr>
+where
+    F: FnOnce() -> Option<SyntaxText>,
+{
+    try_expr_from_text(&format!("{}.{}()", caller()?, text))
+}
 fn expr_from_text(text: &str) -> ast::Expr {
     ast_from_text(&format!("const C: () = {};", text))
 }
diff --git a/docs/dev/style.md b/docs/dev/style.md
index 44f0956c2..bb99c4855 100644
--- a/docs/dev/style.md
+++ b/docs/dev/style.md
@@ -181,6 +181,30 @@ fn frobnicate(walrus: Option<Walrus>) {
 }
 ```
 
+# Early Returns
+
+Do use early returns
+
+```rust
+// Good
+fn foo() -> Option<Bar> {
+    if !condition() {
+        return None;
+    }
+
+    Some(...)
+}
+
+// Not as good
+fn foo() -> Option<Bar> {
+    if condition() {
+        Some(...)
+    } else {
+        None
+    }
+}
+```
+
 # Getters & Setters
 
 If a field can have any value without breaking invariants, make the field public.
@@ -189,7 +213,7 @@ Never provide setters.
 
 Getters should return borrowed data:
 
-```
+```rust
 struct Person {
     // Invariant: never empty
     first_name: String,
@@ -231,6 +255,41 @@ if words.len() != 2 {
 }
 ```
 
+# Avoid Monomorphization
+
+Rust uses monomorphization to compile generic code, meaning that for each instantiation of a generic functions with concrete types, the function is compiled afresh, *per crate*.
+This allows for exceptionally good performance, but leads to increased compile times.
+Runtime performance obeys 80%/20% rule -- only a small fraction of code is hot.
+Compile time **does not** obey this rule -- all code has to be compiled.
+For this reason, avoid making a lot of code type parametric, *especially* on the boundaries between crates.
+
+```rust
+// Good
+fn frbonicate(f: impl FnMut()) {
+    frobnicate_impl(&mut f)
+}
+fn frobnicate_impl(f: &mut dyn FnMut()) {
+    // lots of code
+}
+
+// Not as good
+fn frbonicate(f: impl FnMut()) {
+    // lots of code
+}
+```
+
+Avoid `AsRef` polymorphism, it pays back only for widely used libraries:
+
+```rust
+// Good
+fn frbonicate(f: &Path) {
+}
+
+// Not as good
+fn frbonicate(f: impl AsRef<Path>) {
+}
+```
+
 # Documentation
 
 For `.md` and `.adoc` files, prefer a sentence-per-line format, don't wrap lines.