Rollup merge of #84313 - lcnr:sized-err-msg, r=petrochenkov

fix suggestion for unsized function parameters

taken from `@fasterthanlime's` article https://fasterthanli.me/articles/whats-in-the-box

Author: GuillaumeGomez

compiler/rustc_typeck/src/check/gather_locals.rs

@@ -6,20 +6,19 @@ use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKi
 use rustc_middle::ty::Ty;
 use rustc_span::{sym, Span};
 use rustc_trait_selection::traits;
-use std::mem;
 
 pub(super) struct GatherLocalsVisitor<'a, 'tcx> {
     fcx: &'a FnCtxt<'a, 'tcx>,
     parent_id: hir::HirId,
     // parameters are special cases of patterns, but we want to handle them as
     // *distinct* cases. so track when we are hitting a pattern *within* an fn
     // parameter.
-    outermost_fn_param_pat: bool,
+    outermost_fn_param_pat: Option<Span>,
 }
 
 impl<'a, 'tcx> GatherLocalsVisitor<'a, 'tcx> {
     pub(super) fn new(fcx: &'a FnCtxt<'a, 'tcx>, parent_id: hir::HirId) -> Self {
-        Self { fcx, parent_id, outermost_fn_param_pat: false }
+        Self { fcx, parent_id, outermost_fn_param_pat: None }
     }
 
     fn assign(&mut self, span: Span, nid: hir::HirId, ty_opt: Option<LocalTy<'tcx>>) -> Ty<'tcx> {
@@ -92,7 +91,7 @@ impl<'a, 'tcx> Visitor<'tcx> for GatherLocalsVisitor<'a, 'tcx> {
     }
 
     fn visit_param(&mut self, param: &'tcx hir::Param<'tcx>) {
-        let old_outermost_fn_param_pat = mem::replace(&mut self.outermost_fn_param_pat, true);
+        let old_outermost_fn_param_pat = self.outermost_fn_param_pat.replace(param.ty_span);
         intravisit::walk_param(self, param);
         self.outermost_fn_param_pat = old_outermost_fn_param_pat;
     }
@@ -102,12 +101,12 @@ impl<'a, 'tcx> Visitor<'tcx> for GatherLocalsVisitor<'a, 'tcx> {
         if let PatKind::Binding(_, _, ident, _) = p.kind {
             let var_ty = self.assign(p.span, p.hir_id, None);
 
-            if self.outermost_fn_param_pat {
+            if let Some(ty_span) = self.outermost_fn_param_pat {
                 if !self.fcx.tcx.features().unsized_fn_params {
                     self.fcx.require_type_is_sized(
                         var_ty,
                         p.span,
-                        traits::SizedArgumentType(Some(p.span)),
+                        traits::SizedArgumentType(Some(ty_span)),
                     );
                 }
             } else {
@@ -123,7 +122,7 @@ impl<'a, 'tcx> Visitor<'tcx> for GatherLocalsVisitor<'a, 'tcx> {
                 var_ty
             );
         }
-        let old_outermost_fn_param_pat = mem::replace(&mut self.outermost_fn_param_pat, false);
+        let old_outermost_fn_param_pat = self.outermost_fn_param_pat.take();
         intravisit::walk_pat(self, p);
         self.outermost_fn_param_pat = old_outermost_fn_param_pat;
     }

src/test/ui/error-codes/E0277.stderr

@@ -9,8 +9,8 @@ LL | fn f(p: Path) { }
    = help: unsized fn params are gated as an unstable feature
 help: function arguments must have a statically known size, borrowed types always have a known size
    |
-LL | fn f(&p: Path) { }
-   |      ^
+LL | fn f(p: &Path) { }
+   |         ^
 
 error[E0277]: the trait bound `i32: Foo` is not satisfied
   --> $DIR/E0277.rs:15:15

src/test/ui/feature-gates/feature-gate-unsized_fn_params.stderr

@@ -8,8 +8,8 @@ LL | fn foo(x: dyn Foo) {
    = help: unsized fn params are gated as an unstable feature
 help: function arguments must have a statically known size, borrowed types always have a known size
    |
-LL | fn foo(&x: dyn Foo) {
-   |        ^
+LL | fn foo(x: &dyn Foo) {
+   |           ^
 
 error[E0277]: the size for values of type `(dyn Foo + 'static)` cannot be known at compilation time
   --> $DIR/feature-gate-unsized_fn_params.rs:24:5

src/test/ui/feature-gates/feature-gate-unsized_locals.stderr

@@ -8,8 +8,8 @@ LL | fn f(f: dyn FnOnce()) {}
    = help: unsized fn params are gated as an unstable feature
 help: function arguments must have a statically known size, borrowed types always have a known size
    |
-LL | fn f(&f: dyn FnOnce()) {}
-   |      ^
+LL | fn f(f: &dyn FnOnce()) {}
+   |         ^
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-5883.rs

@@ -4,9 +4,9 @@ struct Struct {
     r: dyn A + 'static
 }
 
-fn new_struct(r: dyn A + 'static)
-    -> Struct { //~^ ERROR the size for values of type
-    //~^ ERROR the size for values of type
+fn new_struct(
+    r: dyn A + 'static //~ ERROR the size for values of type
+) -> Struct {          //~ ERROR the size for values of type
     Struct { r: r }
 }
 

src/test/ui/issues/issue-5883.stderr

@@ -1,22 +1,21 @@
 error[E0277]: the size for values of type `(dyn A + 'static)` cannot be known at compilation time
-  --> $DIR/issue-5883.rs:7:15
+  --> $DIR/issue-5883.rs:8:5
    |
-LL | fn new_struct(r: dyn A + 'static)
-   |               ^ doesn't have a size known at compile-time
+LL |     r: dyn A + 'static
+   |     ^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `(dyn A + 'static)`
    = help: unsized fn params are gated as an unstable feature
 help: function arguments must have a statically known size, borrowed types always have a known size
    |
-LL | fn new_struct(&r: dyn A + 'static)
-   |               ^
+LL |     r: &dyn A + 'static
+   |        ^
 
 error[E0277]: the size for values of type `(dyn A + 'static)` cannot be known at compilation time
-  --> $DIR/issue-5883.rs:8:8
+  --> $DIR/issue-5883.rs:9:6
    |
-LL |     -> Struct {
-   |        ^^^^^^ doesn't have a size known at compile-time
-LL |
+LL | ) -> Struct {
+   |      ^^^^^^ doesn't have a size known at compile-time
 LL |     Struct { r: r }
    |     --------------- this returned value is of type `Struct`
    |

src/test/ui/resolve/issue-5035-2.stderr

@@ -8,8 +8,8 @@ LL | fn foo(_x: K) {}
    = help: unsized fn params are gated as an unstable feature
 help: function arguments must have a statically known size, borrowed types always have a known size
    |
-LL | fn foo(&_x: K) {}
-   |        ^
+LL | fn foo(_x: &K) {}
+   |            ^
 
 error: aborting due to previous error
 

src/test/ui/suggestions/path-by-value.stderr

@@ -9,8 +9,8 @@ LL | fn f(p: Path) { }
    = help: unsized fn params are gated as an unstable feature
 help: function arguments must have a statically known size, borrowed types always have a known size
    |
-LL | fn f(&p: Path) { }
-   |      ^
+LL | fn f(p: &Path) { }
+   |         ^
 
 error: aborting due to previous error
 

src/test/ui/traits/bound/not-on-bare-trait.stderr

@@ -16,8 +16,8 @@ LL | fn foo(_x: Foo + Send) {
    = help: unsized fn params are gated as an unstable feature
 help: function arguments must have a statically known size, borrowed types always have a known size
    |
-LL | fn foo(&_x: Foo + Send) {
-   |        ^
+LL | fn foo(_x: &Foo + Send) {
+   |            ^
 
 error: aborting due to previous error; 1 warning emitted
 

src/test/ui/unsized/unsized-fn-arg.fixed

@@ -0,0 +1,6 @@
+// run-rustfix
+#![crate_type="lib"]
+#![allow(unused)]
+
+fn f<T: ?Sized>(t: &T) {}
+//~^ ERROR the size for values of type `T` cannot be known at compilation time