Create an assist to convert closure to freestanding fn

The assist converts all captures to parameters.

crates/hir-ty/src/infer/closure.rs

@@ -19,7 +19,8 @@ use hir_expand::name::Name;
 use intern::sym;
 use rustc_hash::FxHashMap;
 use smallvec::{smallvec, SmallVec};
-use stdx::never;
+use stdx::{format_to, never};
+use syntax::utils::is_raw_identifier;
 
 use crate::{
     db::{HirDatabase, InternedClosure},
@@ -251,6 +252,11 @@ impl CapturedItem {
         self.place.local
     }
 
+    /// Returns whether this place has any field (aka. non-deref) projections.
+    pub fn has_field_projections(&self) -> bool {
+        self.place.projections.iter().any(|it| !matches!(it, ProjectionElem::Deref))
+    }
+
     pub fn ty(&self, subst: &Substitution) -> Ty {
         self.ty.clone().substitute(Interner, utils::ClosureSubst(subst).parent_subst())
     }
@@ -263,6 +269,97 @@ impl CapturedItem {
         self.span_stacks.iter().map(|stack| *stack.last().expect("empty span stack")).collect()
     }
 
+    /// Converts the place to a name that can be inserted into source code.
+    pub fn place_to_name(&self, owner: DefWithBodyId, db: &dyn HirDatabase) -> String {
+        let body = db.body(owner);
+        let mut result = body[self.place.local].name.unescaped().display(db.upcast()).to_string();
+        for proj in &self.place.projections {
+            match proj {
+                ProjectionElem::Deref => {}
+                ProjectionElem::Field(Either::Left(f)) => {
+                    match &*f.parent.variant_data(db.upcast()) {
+                        VariantData::Record(fields) => {
+                            result.push('_');
+                            result.push_str(fields[f.local_id].name.as_str())
+                        }
+                        VariantData::Tuple(fields) => {
+                            let index = fields.iter().position(|it| it.0 == f.local_id);
+                            if let Some(index) = index {
+                                format_to!(result, "_{index}");
+                            }
+                        }
+                        VariantData::Unit => {}
+                    }
+                }
+                ProjectionElem::Field(Either::Right(f)) => format_to!(result, "_{}", f.index),
+                &ProjectionElem::ClosureField(field) => format_to!(result, "_{field}"),
+                ProjectionElem::Index(_)
+                | ProjectionElem::ConstantIndex { .. }
+                | ProjectionElem::Subslice { .. }
+                | ProjectionElem::OpaqueCast(_) => {
+                    never!("Not happen in closure capture");
+                    continue;
+                }
+            }
+        }
+        if is_raw_identifier(&result, db.crate_graph()[owner.module(db.upcast()).krate()].edition) {
+            result.insert_str(0, "r#");
+        }
+        result
+    }
+
+    pub fn display_place_source_code(&self, owner: DefWithBodyId, db: &dyn HirDatabase) -> String {
+        let body = db.body(owner);
+        let krate = owner.krate(db.upcast());
+        let edition = db.crate_graph()[krate].edition;
+        let mut result = body[self.place.local].name.display(db.upcast(), edition).to_string();
+        for proj in &self.place.projections {
+            match proj {
+                // In source code autoderef kicks in.
+                ProjectionElem::Deref => {}
+                ProjectionElem::Field(Either::Left(f)) => {
+                    let variant_data = f.parent.variant_data(db.upcast());
+                    match &*variant_data {
+                        VariantData::Record(fields) => format_to!(
+                            result,
+                            ".{}",
+                            fields[f.local_id].name.display(db.upcast(), edition)
+                        ),
+                        VariantData::Tuple(fields) => format_to!(
+                            result,
+                            ".{}",
+                            fields.iter().position(|it| it.0 == f.local_id).unwrap_or_default()
+                        ),
+                        VariantData::Unit => {}
+                    }
+                }
+                ProjectionElem::Field(Either::Right(f)) => {
+                    let field = f.index;
+                    format_to!(result, ".{field}");
+                }
+                &ProjectionElem::ClosureField(field) => {
+                    format_to!(result, ".{field}");
+                }
+                ProjectionElem::Index(_)
+                | ProjectionElem::ConstantIndex { .. }
+                | ProjectionElem::Subslice { .. }
+                | ProjectionElem::OpaqueCast(_) => {
+                    never!("Not happen in closure capture");
+                    continue;
+                }
+            }
+        }
+        let final_derefs_count = self
+            .place
+            .projections
+            .iter()
+            .rev()
+            .take_while(|proj| matches!(proj, ProjectionElem::Deref))
+            .count();
+        result.insert_str(0, &"*".repeat(final_derefs_count));
+        result
+    }
+
     pub fn display_place(&self, owner: DefWithBodyId, db: &dyn HirDatabase) -> String {
         let body = db.body(owner);
         let krate = owner.krate(db.upcast());
@@ -451,14 +548,6 @@ impl InferenceContext<'_> {
         });
     }
 
-    fn is_ref_span(&self, span: MirSpan) -> bool {
-        match span {
-            MirSpan::ExprId(expr) => matches!(self.body[expr], Expr::Ref { .. }),
-            MirSpan::BindingId(_) => true,
-            MirSpan::PatId(_) | MirSpan::SelfParam | MirSpan::Unknown => false,
-        }
-    }
-
     fn truncate_capture_spans(&self, capture: &mut CapturedItemWithoutTy, mut truncate_to: usize) {
         // The first span is the identifier, and it must always remain.
         truncate_to += 1;
@@ -467,7 +556,7 @@ impl InferenceContext<'_> {
             let mut actual_truncate_to = 0;
             for &span in &*span_stack {
                 actual_truncate_to += 1;
-                if !self.is_ref_span(span) {
+                if !span.is_ref_span(self.body) {
                     remained -= 1;
                     if remained == 0 {
                         break;
@@ -475,7 +564,7 @@ impl InferenceContext<'_> {
                 }
             }
             if actual_truncate_to < span_stack.len()
-                && self.is_ref_span(span_stack[actual_truncate_to])
+                && span_stack[actual_truncate_to].is_ref_span(self.body)
             {
                 // Include the ref operator if there is one, we will fix it later (in `strip_captures_ref_span()`) if it's incorrect.
                 actual_truncate_to += 1;
@@ -1147,7 +1236,7 @@ impl InferenceContext<'_> {
         for capture in &mut captures {
             if matches!(capture.kind, CaptureKind::ByValue) {
                 for span_stack in &mut capture.span_stacks {
-                    if self.is_ref_span(span_stack[span_stack.len() - 1]) {
+                    if span_stack[span_stack.len() - 1].is_ref_span(self.body) {
                         span_stack.truncate(span_stack.len() - 1);
                     }
                 }

crates/hir-ty/src/mir.rs

@@ -16,7 +16,8 @@ use base_db::CrateId;
 use chalk_ir::Mutability;
 use either::Either;
 use hir_def::{
-    hir::{BindingId, Expr, ExprId, Ordering, PatId},
+    body::Body,
+    hir::{BindingAnnotation, BindingId, Expr, ExprId, Ordering, PatId},
     DefWithBodyId, FieldId, StaticId, TupleFieldId, UnionId, VariantId,
 };
 use la_arena::{Arena, ArenaMap, Idx, RawIdx};
@@ -1174,6 +1175,20 @@ pub enum MirSpan {
     Unknown,
 }
 
+impl MirSpan {
+    pub fn is_ref_span(&self, body: &Body) -> bool {
+        match *self {
+            MirSpan::ExprId(expr) => matches!(body[expr], Expr::Ref { .. }),
+            // FIXME: Figure out if this is correct wrt. match ergonomics.
+            MirSpan::BindingId(binding) => matches!(
+                body.bindings[binding].mode,
+                BindingAnnotation::Ref | BindingAnnotation::RefMut
+            ),
+            MirSpan::PatId(_) | MirSpan::SelfParam | MirSpan::Unknown => false,
+        }
+    }
+}
+
 impl_from!(ExprId, PatId for MirSpan);
 
 impl From<&ExprId> for MirSpan {

crates/hir/src/lib.rs

@@ -78,6 +78,7 @@ use hir_ty::{
 use itertools::Itertools;
 use nameres::diagnostics::DefDiagnosticKind;
 use rustc_hash::FxHashSet;
+use smallvec::SmallVec;
 use span::{Edition, EditionedFileId, FileId, MacroCallId, SyntaxContextId};
 use stdx::{impl_from, never};
 use syntax::{
@@ -4113,6 +4114,15 @@ impl ClosureCapture {
         Local { parent: self.owner, binding_id: self.capture.local() }
     }
 
+    /// Returns whether this place has any field (aka. non-deref) projections.
+    pub fn has_field_projections(&self) -> bool {
+        self.capture.has_field_projections()
+    }
+
+    pub fn usages(&self) -> CaptureUsages {
+        CaptureUsages { parent: self.owner, spans: self.capture.spans() }
+    }
+
     pub fn kind(&self) -> CaptureKind {
         match self.capture.kind() {
             hir_ty::CaptureKind::ByRef(
@@ -4128,6 +4138,15 @@ impl ClosureCapture {
         }
     }
 
+    /// Converts the place to a name that can be inserted into source code.
+    pub fn place_to_name(&self, db: &dyn HirDatabase) -> String {
+        self.capture.place_to_name(self.owner, db)
+    }
+
+    pub fn display_place_source_code(&self, db: &dyn HirDatabase) -> String {
+        self.capture.display_place_source_code(self.owner, db)
+    }
+
     pub fn display_place(&self, db: &dyn HirDatabase) -> String {
         self.capture.display_place(self.owner, db)
     }
@@ -4141,6 +4160,74 @@ pub enum CaptureKind {
     Move,
 }
 
+#[derive(Debug, Clone)]
+pub struct CaptureUsages {
+    parent: DefWithBodyId,
+    spans: SmallVec<[mir::MirSpan; 3]>,
+}
+
+impl CaptureUsages {
+    pub fn sources(&self, db: &dyn HirDatabase) -> Vec<CaptureUsageSource> {
+        let (body, source_map) = db.body_with_source_map(self.parent);
+        let mut result = Vec::with_capacity(self.spans.len());
+        for &span in self.spans.iter() {
+            let is_ref = span.is_ref_span(&body);
+            match span {
+                mir::MirSpan::ExprId(expr) => {
+                    if let Ok(expr) = source_map.expr_syntax(expr) {
+                        result.push(CaptureUsageSource {
+                            is_ref,
+                            source: expr.map(AstPtr::wrap_left),
+                        })
+                    }
+                }
+                mir::MirSpan::PatId(pat) => {
+                    if let Ok(pat) = source_map.pat_syntax(pat) {
+                        result.push(CaptureUsageSource {
+                            is_ref,
+                            source: pat.map(AstPtr::wrap_right),
+                        });
+                    }
+                }
+                mir::MirSpan::BindingId(binding) => result.extend(
+                    source_map
+                        .patterns_for_binding(binding)
+                        .iter()
+                        .filter_map(|&pat| source_map.pat_syntax(pat).ok())
+                        .map(|pat| CaptureUsageSource {
+                            is_ref,
+                            source: pat.map(AstPtr::wrap_right),
+                        }),
+                ),
+                mir::MirSpan::SelfParam | mir::MirSpan::Unknown => {
+                    unreachable!("invalid capture usage span")
+                }
+            }
+        }
+        result
+    }
+}
+
+#[derive(Debug)]
+pub struct CaptureUsageSource {
+    is_ref: bool,
+    source: InFile<AstPtr<Either<ast::Expr, ast::Pat>>>,
+}
+
+impl CaptureUsageSource {
+    pub fn source(&self) -> AstPtr<Either<ast::Expr, ast::Pat>> {
+        self.source.value
+    }
+
+    pub fn file_id(&self) -> HirFileId {
+        self.source.file_id
+    }
+
+    pub fn is_ref(&self) -> bool {
+        self.is_ref
+    }
+}
+
 #[derive(Clone, PartialEq, Eq, Debug, Hash)]
 pub struct Type {
     env: Arc<TraitEnvironment>,

crates/ide-assists/src/lib.rs

@@ -116,6 +116,7 @@ mod handlers {
     mod bool_to_enum;
     mod change_visibility;
     mod convert_bool_then;
+    mod convert_closure_to_fn;
     mod convert_comment_block;
     mod convert_comment_from_or_to_doc;
     mod convert_from_to_tryfrom;
@@ -245,6 +246,7 @@ mod handlers {
             toggle_async_sugar::sugar_impl_future_into_async,
             convert_comment_block::convert_comment_block,
             convert_comment_from_or_to_doc::convert_comment_from_or_to_doc,
+            convert_closure_to_fn::convert_closure_to_fn,
             convert_from_to_tryfrom::convert_from_to_tryfrom,
             convert_integer_literal::convert_integer_literal,
             convert_into_to_from::convert_into_to_from,

crates/ide-assists/src/tests/generated.rs

@@ -382,6 +382,40 @@ fn main() {
     )
 }
 
+#[test]
+fn doctest_convert_closure_to_fn() {
+    check_doc_test(
+        "convert_closure_to_fn",
+        r#####"
+//- minicore: copy
+struct String;
+impl String {
+    fn new() -> Self {}
+    fn push_str(&mut self, s: &str) {}
+}
+fn main() {
+    let mut s = String::new();
+    let closure = |$0a| s.push_str(a);
+    closure("abc");
+}
+"#####,
+        r#####"
+struct String;
+impl String {
+    fn new() -> Self {}
+    fn push_str(&mut self, s: &str) {}
+}
+fn main() {
+    let mut s = String::new();
+    fn closure(a: &str, s: &mut String) {
+        s.push_str(a)
+    }
+    closure("abc", &mut s);
+}
+"#####,
+    )
+}
+
 #[test]
 fn doctest_convert_for_loop_with_for_each() {
     check_doc_test(