Rollup merge of rust-lang#97964 - WaffleLapkin:fix_borrow_par_suggest…

…ions, r=compiler-errors

Fix suggestions for `&a: T` parameters

I've accidentally discovered that we have broken suggestions for `&a: T` parameters:
```rust
fn f(&mut bar: u32) {}

fn main() {
    let _ = |&mut a| ();
}
```
```text
error[E0308]: mismatched types
 --> ./t.rs:1:6
  |
1 | fn f(&mut bar: u32) {}
  |      ^^^^^^^^-----
  |      |         |
  |      |         expected due to this
  |      expected `u32`, found `&mut _`
  |      help: did you mean `bar`: `&u32`
  |
  = note:           expected type `u32`
          found mutable reference `&mut _`

error[E0308]: mismatched types
 --> ./t.rs:4:23
  |
4 |     let _: fn(u32) = |&mut a| ();
  |                       ^^^^^--
  |                       |    |
  |                       |    expected due to this
  |                       expected `u32`, found `&mut _`
  |                       help: did you mean `a`: `&u32`
  |
  = note:           expected type `u32`
          found mutable reference `&mut _`
```

It's hard to see, but
1. The help span is overlapping with "expected" spans
2. It suggests `fn f( &u32) {}` (no `mut` and lost parameter name) and `|&u32 ()` (no closing `|` and lost parameter name)

I've tried to fix this.

r? `@compiler-errors`

compiler/rustc_typeck/src/check/pat.rs

@@ -649,39 +649,41 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
-    fn borrow_pat_suggestion(
-        &self,
-        err: &mut Diagnostic,
-        pat: &Pat<'_>,
-        inner: &Pat<'_>,
-        expected: Ty<'tcx>,
-    ) {
+    // Precondition: pat is a Ref(_) pattern
+    fn borrow_pat_suggestion(&self, err: &mut Diagnostic, pat: &Pat<'_>) {
         let tcx = self.tcx;
-        if let PatKind::Binding(..) = inner.kind {
+        if let PatKind::Ref(inner, mutbl) = pat.kind
+        && let PatKind::Binding(_, _, binding, ..) = inner.kind {
             let binding_parent_id = tcx.hir().get_parent_node(pat.hir_id);
             let binding_parent = tcx.hir().get(binding_parent_id);
-            debug!("inner {:?} pat {:?} parent {:?}", inner, pat, binding_parent);
+            debug!(?inner, ?pat, ?binding_parent);
+
+            let mutability = match mutbl {
+                ast::Mutability::Mut => "mut",
+                ast::Mutability::Not => "",
+            };
+
             match binding_parent {
-                hir::Node::Param(hir::Param { span, .. })
-                    if let Ok(snippet) = tcx.sess.source_map().span_to_snippet(inner.span) =>
-                {
-                    err.span_suggestion(
-                        *span,
-                        &format!("did you mean `{snippet}`"),
-                        format!(" &{expected}"),
-                        Applicability::MachineApplicable,
+                // Check that there is explicit type (ie this is not a closure param with inferred type)
+                // so we don't suggest moving something to the type that does not exist
+                hir::Node::Param(hir::Param { ty_span, .. }) if binding.span != *ty_span => {
+                    err.multipart_suggestion_verbose(
+                        format!("to take parameter `{binding}` by reference, move `&{mutability}` to the type"),
+                        vec![
+                            (pat.span.until(inner.span), "".to_owned()),
+                            (ty_span.shrink_to_lo(), format!("&{}", mutbl.prefix_str())),
+                        ],
+                        Applicability::MachineApplicable
                     );
                 }
-                hir::Node::Arm(_) | hir::Node::Pat(_) => {
+                hir::Node::Param(_) | hir::Node::Arm(_) | hir::Node::Pat(_) => {
                     // rely on match ergonomics or it might be nested `&&pat`
-                    if let Ok(snippet) = tcx.sess.source_map().span_to_snippet(inner.span) {
-                        err.span_suggestion(
-                            pat.span,
-                            "you can probably remove the explicit borrow",
-                            snippet,
-                            Applicability::MaybeIncorrect,
-                        );
-                    }
+                    err.span_suggestion_verbose(
+                        pat.span.until(inner.span),
+                        format!("consider removing `&{mutability}` from the pattern"),
+                        "",
+                        Applicability::MaybeIncorrect,
+                    );
                 }
                 _ => {} // don't provide suggestions in other cases #55175
             }
@@ -1836,6 +1838,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         box_ty
     }
 
+    // Precondition: Pat is Ref(inner)
     fn check_pat_ref(
         &self,
         pat: &'tcx Pat<'tcx>,
@@ -1853,7 +1856,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
             // Take region, inner-type from expected type if we can,
             // to avoid creating needless variables. This also helps with
-            // the bad  interactions of the given hack detailed in (note_1).
+            // the bad interactions of the given hack detailed in (note_1).
             debug!("check_pat_ref: expected={:?}", expected);
             match *expected.kind() {
                 ty::Ref(_, r_ty, r_mutbl) if r_mutbl == mutbl => (expected, r_ty),
@@ -1869,7 +1872,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     // Look for a case like `fn foo(&foo: u32)` and suggest
                     // `fn foo(foo: &u32)`
                     if let Some(mut err) = err {
-                        self.borrow_pat_suggestion(&mut err, pat, inner, expected);
+                        self.borrow_pat_suggestion(&mut err, pat);
                         err.emit();
                     }
                     (rptr_ty, inner_ty)

src/test/ui/destructure-trait-ref.stderr

@@ -23,10 +23,14 @@ LL |     let &&x = &1isize as &dyn T;
    |          ^^   ----------------- this expression has type `&dyn T`
    |          |
    |          expected trait object `dyn T`, found reference
-   |          help: you can probably remove the explicit borrow: `x`
    |
    = note: expected trait object `dyn T`
                  found reference `&_`
+help: consider removing `&` from the pattern
+   |
+LL -     let &&x = &1isize as &dyn T;
+LL +     let &x = &1isize as &dyn T;
+   | 
 
 error[E0308]: mismatched types
   --> $DIR/destructure-trait-ref.rs:36:11
@@ -35,10 +39,14 @@ LL |     let &&&x = &(&1isize as &dyn T);
    |           ^^   -------------------- this expression has type `&&dyn T`
    |           |
    |           expected trait object `dyn T`, found reference
-   |           help: you can probably remove the explicit borrow: `x`
    |
    = note: expected trait object `dyn T`
                  found reference `&_`
+help: consider removing `&` from the pattern
+   |
+LL -     let &&&x = &(&1isize as &dyn T);
+LL +     let &&x = &(&1isize as &dyn T);
+   | 
 
 error[E0308]: mismatched types
   --> $DIR/destructure-trait-ref.rs:40:13

src/test/ui/mismatched_types/issue-38371-unfixable.rs

@@ -0,0 +1,5 @@
+fn ugh(&[bar]: &u32) {} //~ ERROR expected an array or slice
+
+fn bgh(&&bar: u32) {} //~ ERROR mismatched types
+
+fn main() {}

src/test/ui/mismatched_types/issue-38371-unfixable.stderr

@@ -0,0 +1,21 @@
+error[E0529]: expected an array or slice, found `u32`
+  --> $DIR/issue-38371-unfixable.rs:1:9
+   |
+LL | fn ugh(&[bar]: &u32) {}
+   |         ^^^^^ pattern cannot match with input type `u32`
+
+error[E0308]: mismatched types
+  --> $DIR/issue-38371-unfixable.rs:3:8
+   |
+LL | fn bgh(&&bar: u32) {}
+   |        ^^^^^  --- expected due to this
+   |        |
+   |        expected `u32`, found reference
+   |
+   = note:   expected type `u32`
+           found reference `&_`
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0308, E0529.
+For more information about an error, try `rustc --explain E0308`.

src/test/ui/mismatched_types/issue-38371.fixed

@@ -0,0 +1,18 @@
+// run-rustfix
+// see also issue-38371-unfixable.rs
+#![allow(dead_code)]
+
+#[derive(Copy, Clone)]
+struct Foo {}
+
+fn foo(_a: &Foo) {} //~ ERROR mismatched types
+
+fn bar(_a: Foo) {}
+
+fn qux(_a: &Foo) {}
+
+fn zar(&_a: &Foo) {}
+
+fn agh(&_a: &u32) {} //~ ERROR mismatched types
+
+fn main() {}

src/test/ui/mismatched_types/issue-38371.rs

@@ -1,27 +1,18 @@
-struct Foo {
-}
+// run-rustfix
+// see also issue-38371-unfixable.rs
+#![allow(dead_code)]
 
-fn foo(&foo: Foo) { //~ ERROR mismatched types
-}
+#[derive(Copy, Clone)]
+struct Foo {}
 
-fn bar(foo: Foo) {
-}
+fn foo(&_a: Foo) {} //~ ERROR mismatched types
 
-fn qux(foo: &Foo) {
-}
+fn bar(_a: Foo) {}
 
-fn zar(&foo: &Foo) {
-}
+fn qux(_a: &Foo) {}
 
-// The somewhat unexpected help message in this case is courtesy of
-// match_default_bindings.
-fn agh(&&bar: &u32) { //~ ERROR mismatched types
-}
+fn zar(&_a: &Foo) {}
 
-fn bgh(&&bar: u32) { //~ ERROR mismatched types
-}
-
-fn ugh(&[bar]: &u32) { //~ ERROR expected an array or slice
-}
+fn agh(&&_a: &u32) {} //~ ERROR mismatched types
 
 fn main() {}

src/test/ui/mismatched_types/issue-38371.stderr

@@ -1,46 +1,35 @@
 error[E0308]: mismatched types
-  --> $DIR/issue-38371.rs:4:8
+  --> $DIR/issue-38371.rs:8:8
    |
-LL | fn foo(&foo: Foo) {
-   |        ^^^^-----
-   |        |     |
-   |        |     expected due to this
+LL | fn foo(&_a: Foo) {}
+   |        ^^^  --- expected due to this
+   |        |
    |        expected struct `Foo`, found reference
-   |        help: did you mean `foo`: `&Foo`
    |
    = note: expected struct `Foo`
            found reference `&_`
+help: to take parameter `_a` by reference, move `&` to the type
+   |
+LL - fn foo(&_a: Foo) {}
+LL + fn foo(_a: &Foo) {}
+   | 
 
 error[E0308]: mismatched types
-  --> $DIR/issue-38371.rs:18:9
+  --> $DIR/issue-38371.rs:16:9
    |
-LL | fn agh(&&bar: &u32) {
-   |         ^^^^  ---- expected due to this
+LL | fn agh(&&_a: &u32) {}
+   |         ^^^  ---- expected due to this
    |         |
    |         expected `u32`, found reference
-   |         help: you can probably remove the explicit borrow: `bar`
-   |
-   = note:   expected type `u32`
-           found reference `&_`
-
-error[E0308]: mismatched types
-  --> $DIR/issue-38371.rs:21:8
-   |
-LL | fn bgh(&&bar: u32) {
-   |        ^^^^^  --- expected due to this
-   |        |
-   |        expected `u32`, found reference
    |
    = note:   expected type `u32`
            found reference `&_`
-
-error[E0529]: expected an array or slice, found `u32`
-  --> $DIR/issue-38371.rs:24:9
+help: consider removing `&` from the pattern
    |
-LL | fn ugh(&[bar]: &u32) {
-   |         ^^^^^ pattern cannot match with input type `u32`
+LL - fn agh(&&_a: &u32) {}
+LL + fn agh(&_a: &u32) {}
+   | 
 
-error: aborting due to 4 previous errors
+error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0308, E0529.
-For more information about an error, try `rustc --explain E0308`.
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/mismatched_types/ref-pat-suggestions.fixed

@@ -0,0 +1,24 @@
+// run-rustfix
+
+fn _f0(_a: &u32) {} //~ ERROR mismatched types
+fn _f1(_a: &mut u32) {} //~ ERROR mismatched types
+fn _f2(&_a: &u32) {} //~ ERROR mismatched types
+fn _f3(&mut _a: &mut u32) {} //~ ERROR mismatched types
+fn _f4(&_a: &u32) {} //~ ERROR mismatched types
+fn _f5(&mut _a: &mut u32) {} //~ ERROR mismatched types
+
+fn main() {
+    let _: fn(u32) = |_a| (); //~ ERROR mismatched types
+    let _: fn(u32) = |_a| (); //~ ERROR mismatched types
+    let _: fn(&u32) = |&_a| (); //~ ERROR mismatched types
+    let _: fn(&mut u32) = |&mut _a| (); //~ ERROR mismatched types
+    let _: fn(&u32) = |&_a| (); //~ ERROR mismatched types
+    let _: fn(&mut u32) = |&mut _a| (); //~ ERROR mismatched types
+
+    let _ = |_a: &u32| (); //~ ERROR mismatched types
+    let _ = |_a: &mut u32| (); //~ ERROR mismatched types
+    let _ = |&_a: &u32| (); //~ ERROR mismatched types
+    let _ = |&mut _a: &mut u32| (); //~ ERROR mismatched types
+    let _ = |&_a: &u32| (); //~ ERROR mismatched types
+    let _ = |&mut _a: &mut u32| (); //~ ERROR mismatched types
+}

src/test/ui/mismatched_types/ref-pat-suggestions.rs

@@ -0,0 +1,24 @@
+// run-rustfix
+
+fn _f0(&_a: u32) {} //~ ERROR mismatched types
+fn _f1(&mut _a: u32) {} //~ ERROR mismatched types
+fn _f2(&&_a: &u32) {} //~ ERROR mismatched types
+fn _f3(&mut &_a: &mut u32) {} //~ ERROR mismatched types
+fn _f4(&&mut _a: &u32) {} //~ ERROR mismatched types
+fn _f5(&mut &mut _a: &mut u32) {} //~ ERROR mismatched types
+
+fn main() {
+    let _: fn(u32) = |&_a| (); //~ ERROR mismatched types
+    let _: fn(u32) = |&mut _a| (); //~ ERROR mismatched types
+    let _: fn(&u32) = |&&_a| (); //~ ERROR mismatched types
+    let _: fn(&mut u32) = |&mut &_a| (); //~ ERROR mismatched types
+    let _: fn(&u32) = |&&mut _a| (); //~ ERROR mismatched types
+    let _: fn(&mut u32) = |&mut &mut _a| (); //~ ERROR mismatched types
+
+    let _ = |&_a: u32| (); //~ ERROR mismatched types
+    let _ = |&mut _a: u32| (); //~ ERROR mismatched types
+    let _ = |&&_a: &u32| (); //~ ERROR mismatched types
+    let _ = |&mut &_a: &mut u32| (); //~ ERROR mismatched types
+    let _ = |&&mut _a: &u32| (); //~ ERROR mismatched types
+    let _ = |&mut &mut _a: &mut u32| (); //~ ERROR mismatched types
+}