3 files changed, 227 insertions, 0 deletions

diff --git a/crates/assists/src/handlers/inline_function.rs b/crates/assists/src/handlers/inline_function.rs
new file mode 100644
index 000000000..2edf252b7
--- /dev/null
+++ b/crates/assists/src/handlers/inline_function.rs
@@ -0,0 +1,202 @@
+use ast::make;
+use hir::{HasSource, PathResolution};
+use syntax::{
+    ast::{self, edit::AstNodeEdit, ArgListOwner},
+    AstNode,
+};
+use test_utils::mark;
+
+use crate::{
+    assist_context::{AssistContext, Assists},
+    AssistId, AssistKind,
+};
+
+// Assist: inline_function
+//
+// Inlines a function body.
+//
+// ```
+// fn add(a: u32, b: u32) -> u32 { a + b }
+// fn main() {
+//     let x = add<|>(1, 2);
+// }
+// ```
+// ->
+// ```
+// fn add(a: u32, b: u32) -> u32 { a + b }
+// fn main() {
+//     let x = {
+//         let a = 1;
+//         let b = 2;
+//         a + b
+//     };
+// }
+// ```
+pub(crate) fn inline_function(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {
+    let path_expr: ast::PathExpr = ctx.find_node_at_offset()?;
+    let call = path_expr.syntax().parent().and_then(ast::CallExpr::cast)?;
+    let path = path_expr.path()?;
+
+    let function = match ctx.sema.resolve_path(&path)? {
+        PathResolution::Def(hir::ModuleDef::Function(f)) => f,
+        _ => return None,
+    };
+
+    let function_source = function.source(ctx.db())?;
+    let arguments: Vec<_> = call.arg_list()?.args().collect();
+    let parameters = function_parameter_patterns(&function_source.value)?;
+
+    if arguments.len() != parameters.len() {
+        // Can't inline the function because they've passed the wrong number of
+        // arguments to this function
+        mark::hit!(inline_function_incorrect_number_of_arguments);
+        return None;
+    }
+
+    let new_bindings = parameters.into_iter().zip(arguments);
+
+    let body = function_source.value.body()?;
+
+    acc.add(
+        AssistId("inline_function", AssistKind::RefactorInline),
+        format!("Inline `{}`", path),
+        call.syntax().text_range(),
+        |builder| {
+            let mut statements: Vec<ast::Stmt> = Vec::new();
+
+            for (pattern, value) in new_bindings {
+                statements.push(make::let_stmt(pattern, Some(value)).into());
+            }
+
+            statements.extend(body.statements());
+
+            let original_indentation = call.indent_level();
+            let replacement = make::block_expr(statements, body.expr())
+                .reset_indent()
+                .indent(original_indentation);
+
+            builder.replace_ast(ast::Expr::CallExpr(call), ast::Expr::BlockExpr(replacement));
+        },
+    )
+}
+
+fn function_parameter_patterns(value: &ast::Fn) -> Option<Vec<ast::Pat>> {
+    let mut patterns = Vec::new();
+
+    for param in value.param_list()?.params() {
+        let pattern = param.pat()?;
+        patterns.push(pattern);
+    }
+
+    Some(patterns)
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::tests::{check_assist, check_assist_not_applicable};
+
+    use super::*;
+
+    #[test]
+    fn no_args_or_return_value_gets_inlined_without_block() {
+        check_assist(
+            inline_function,
+            r#"
+fn foo() { println!("Hello, World!"); }
+fn main() {
+    fo<|>o();
+}
+"#,
+            r#"
+fn foo() { println!("Hello, World!"); }
+fn main() {
+    {
+        println!("Hello, World!");
+    };
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn args_with_side_effects() {
+        check_assist(
+            inline_function,
+            r#"
+fn foo(name: String) { println!("Hello, {}!", name); }
+fn main() {
+    foo<|>(String::from("Michael"));
+}
+"#,
+            r#"
+fn foo(name: String) { println!("Hello, {}!", name); }
+fn main() {
+    {
+        let name = String::from("Michael");
+        println!("Hello, {}!", name);
+    };
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn method_inlining_isnt_supported() {
+        check_assist_not_applicable(
+            inline_function,
+            r"
+struct Foo;
+impl Foo { fn bar(&self) {} }
+
+fn main() { Foo.bar<|>(); }
+",
+        );
+    }
+
+    #[test]
+    fn not_applicable_when_incorrect_number_of_parameters_are_provided() {
+        mark::check!(inline_function_incorrect_number_of_arguments);
+        check_assist_not_applicable(
+            inline_function,
+            r#"
+fn add(a: u32, b: u32) -> u32 { a + b }
+fn main() { let x = add<|>(42); }
+"#,
+        );
+    }
+
+    #[test]
+    fn function_with_multiple_statements() {
+        check_assist(
+            inline_function,
+            r#"
+fn foo(a: u32, b: u32) -> u32 {
+    let x = a + b;
+    let y = x - b;
+    x * y
+}
+
+fn main() {
+    let x = foo<|>(1, 2);
+}
+"#,
+            r#"
+fn foo(a: u32, b: u32) -> u32 {
+    let x = a + b;
+    let y = x - b;
+    x * y
+}
+
+fn main() {
+    let x = {
+        let a = 1;
+        let b = 2;
+        let x = a + b;
+        let y = x - b;
+        x * y
+    };
+}
+"#,
+        );
+    }
+}
diff --git a/crates/assists/src/lib.rs b/crates/assists/src/lib.rs
index 01baa65fe..9c2a95735 100644
--- a/crates/assists/src/lib.rs
+++ b/crates/assists/src/lib.rs
@@ -131,6 +131,7 @@ mod handlers {
     mod generate_impl;
     mod generate_new;
     mod infer_function_return_type;
+    mod inline_function;
     mod inline_local_variable;
     mod introduce_named_lifetime;
     mod invert_if;
@@ -183,6 +184,7 @@ mod handlers {
             generate_impl::generate_impl,
             generate_new::generate_new,
             infer_function_return_type::infer_function_return_type,
+            inline_function::inline_function,
             inline_local_variable::inline_local_variable,
             introduce_named_lifetime::introduce_named_lifetime,
             invert_if::invert_if,
diff --git a/crates/assists/src/tests/generated.rs b/crates/assists/src/tests/generated.rs
index 85e3c6742..b15352cf3 100644
--- a/crates/assists/src/tests/generated.rs
+++ b/crates/assists/src/tests/generated.rs
@@ -531,6 +531,29 @@ fn foo() -> i32 { 42i32 }
 }
 
 #[test]
+fn doctest_inline_function() {
+    check_doc_test(
+        "inline_function",
+        r#####"
+fn add(a: u32, b: u32) -> u32 { a + b }
+fn main() {
+    let x = add<|>(1, 2);
+}
+"#####,
+        r#####"
+fn add(a: u32, b: u32) -> u32 { a + b }
+fn main() {
+    let x = {
+        let a = 1;
+        let b = 2;
+        a + b
+    };
+}
+"#####,
+    )
+}
+
+#[test]
 fn doctest_inline_local_variable() {
     check_doc_test(
         "inline_local_variable",