Tweak value suggestions in borrowck and hir_analysis

Unify the output of `suggest_assign_value` and `ty_kind_suggestion`.

Ideally we'd make these a single function, but doing so would likely require modify the crate dependency tree.

Author: estebank

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -672,23 +672,21 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         };
 
         let assign_value = match ty.kind() {
+            ty::Never | ty::Error(_) => return,
             ty::Bool => "false",
             ty::Float(_) => "0.0",
             ty::Int(_) | ty::Uint(_) => "0",
-            ty::Never | ty::Error(_) => "",
             ty::Adt(def, _) if Some(def.did()) == tcx.get_diagnostic_item(sym::Vec) => "vec![]",
             ty::Adt(_, _) if implements_default(ty, self.param_env) => "Default::default()",
-            _ => "todo!()",
+            _ => "value",
         };
 
-        if !assign_value.is_empty() {
-            err.span_suggestion_verbose(
-                sugg_span.shrink_to_hi(),
-                "consider assigning a value",
-                format!(" = {assign_value}"),
-                Applicability::MaybeIncorrect,
-            );
-        }
+        err.span_suggestion_verbose(
+            sugg_span.shrink_to_hi(),
+            "consider assigning a value",
+            format!(" = {assign_value}"),
+            Applicability::MaybeIncorrect,
+        );
     }
 
     fn suggest_borrow_fn_like(

compiler/rustc_hir_analysis/src/check/check.rs

@@ -22,7 +22,6 @@ use rustc_middle::ty::{
     AdtDef, ParamEnv, RegionKind, TypeSuperVisitable, TypeVisitable, TypeVisitableExt,
 };
 use rustc_session::lint::builtin::{UNINHABITED_STATIC, UNSUPPORTED_CALLING_CONVENTIONS};
-use rustc_span::symbol::sym;
 use rustc_target::abi::FieldIdx;
 use rustc_trait_selection::traits::error_reporting::on_unimplemented::OnUnimplementedDirective;
 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt as _;

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -91,10 +91,11 @@ use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_middle::ty::{GenericArgs, GenericArgsRef};
 use rustc_session::parse::feature_err;
-use rustc_span::symbol::{kw, Ident};
-use rustc_span::{self, def_id::CRATE_DEF_ID, BytePos, Span, Symbol, DUMMY_SP};
+use rustc_span::symbol::{kw, sym, Ident};
+use rustc_span::{def_id::CRATE_DEF_ID, BytePos, Span, Symbol, DUMMY_SP};
 use rustc_target::abi::VariantIdx;
 use rustc_target::spec::abi::Abi;
+use rustc_trait_selection::infer::InferCtxtExt;
 use rustc_trait_selection::traits::error_reporting::suggestions::ReturnsVisitor;
 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt as _;
 use rustc_trait_selection::traits::ObligationCtxt;
@@ -466,13 +467,24 @@ fn fn_sig_suggestion<'tcx>(
     )
 }
 
-pub fn ty_kind_suggestion(ty: Ty<'_>) -> Option<&'static str> {
+pub fn ty_kind_suggestion<'tcx>(ty: Ty<'tcx>, tcx: TyCtxt<'tcx>) -> Option<&'static str> {
+    let implements_default = |ty| {
+        let Some(default_trait) = tcx.get_diagnostic_item(sym::Default) else {
+            return false;
+        };
+        let infcx = tcx.infer_ctxt().build();
+        infcx
+            .type_implements_trait(default_trait, [ty], ty::ParamEnv::reveal_all())
+            .must_apply_modulo_regions()
+    };
     Some(match ty.kind() {
         ty::Bool => "true",
         ty::Char => "'a'",
         ty::Int(_) | ty::Uint(_) => "42",
         ty::Float(_) => "3.14159",
         ty::Error(_) | ty::Never => return None,
+        ty::Adt(def, _) if Some(def.did()) == tcx.get_diagnostic_item(sym::Vec) => "vec![]",
+        ty::Adt(_, _) if implements_default(ty) => "Default::default()",
         _ => "value",
     })
 }
@@ -511,7 +523,7 @@ fn suggestion_signature<'tcx>(
         }
         ty::AssocKind::Const => {
             let ty = tcx.type_of(assoc.def_id).instantiate_identity();
-            let val = ty_kind_suggestion(ty).unwrap_or("todo!()");
+            let val = ty_kind_suggestion(ty, tcx).unwrap_or("todo!()");
             format!("const {}: {} = {};", assoc.name, ty, val)
         }
     }

compiler/rustc_hir_typeck/src/expr.rs

@@ -694,10 +694,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             );
                             let error = Some(Sorts(ExpectedFound { expected: ty, found: e_ty }));
                             self.annotate_loop_expected_due_to_inference(err, expr, error);
-                            if let Some(val) = ty_kind_suggestion(ty) {
+                            if let Some(val) = ty_kind_suggestion(ty, tcx) {
                                 err.span_suggestion_verbose(
                                     expr.span.shrink_to_hi(),
-                                    "give it a value of the expected type",
+                                    "give the `break` a value of the expected type",
                                     format!(" {val}"),
                                     Applicability::HasPlaceholders,
                                 );

tests/ui/binop/issue-77910-1.stderr

@@ -32,8 +32,8 @@ LL |     xs
    |
 help: consider assigning a value
    |
-LL |     let xs = todo!();
-   |            +++++++++
+LL |     let xs = value;
+   |            +++++++
 
 error: aborting due to 3 previous errors
 

tests/ui/binop/issue-77910-2.stderr

@@ -21,8 +21,8 @@ LL |     xs
    |
 help: consider assigning a value
    |
-LL |     let xs = todo!();
-   |            +++++++++
+LL |     let xs = value;
+   |            +++++++
 
 error: aborting due to 2 previous errors
 

tests/ui/borrowck/borrowck-init-in-fru.stderr

@@ -8,8 +8,8 @@ LL |     origin = Point { x: 10, ..origin };
    |
 help: consider assigning a value
    |
-LL |     let mut origin: Point = todo!();
-   |                           +++++++++
+LL |     let mut origin: Point = value;
+   |                           +++++++
 
 error: aborting due to 1 previous error
 

tests/ui/borrowck/borrowck-uninit-ref-chain.stderr

@@ -8,8 +8,8 @@ LL |     let _y = &**x;
    |
 help: consider assigning a value
    |
-LL |     let x: &&Box<i32> = todo!();
-   |                       +++++++++
+LL |     let x: &&Box<i32> = value;
+   |                       +++++++
 
 error[E0381]: used binding `x` isn't initialized
   --> $DIR/borrowck-uninit-ref-chain.rs:11:14
@@ -21,8 +21,8 @@ LL |     let _y = &**x;
    |
 help: consider assigning a value
    |
-LL |     let x: &&S<i32, i32> = todo!();
-   |                          +++++++++
+LL |     let x: &&S<i32, i32> = value;
+   |                          +++++++
 
 error[E0381]: used binding `x` isn't initialized
   --> $DIR/borrowck-uninit-ref-chain.rs:14:14
@@ -34,8 +34,8 @@ LL |     let _y = &**x;
    |
 help: consider assigning a value
    |
-LL |     let x: &&i32 = todo!();
-   |                  +++++++++
+LL |     let x: &&i32 = value;
+   |                  +++++++
 
 error[E0381]: partially assigned binding `a` isn't fully initialized
   --> $DIR/borrowck-uninit-ref-chain.rs:18:5

tests/ui/borrowck/borrowck-use-in-index-lvalue.stderr

@@ -8,8 +8,8 @@ LL |     w[5] = 0;
    |
 help: consider assigning a value
    |
-LL |     let w: &mut [isize] = todo!();
-   |                         +++++++++
+LL |     let w: &mut [isize] = value;
+   |                         +++++++
 
 error[E0381]: used binding `w` isn't initialized
   --> $DIR/borrowck-use-in-index-lvalue.rs:6:5
@@ -21,8 +21,8 @@ LL |     w[5] = 0;
    |
 help: consider assigning a value
    |
-LL |     let mut w: &mut [isize] = todo!();
-   |                             +++++++++
+LL |     let mut w: &mut [isize] = value;
+   |                             +++++++
 
 error: aborting due to 2 previous errors
 

tests/ui/borrowck/borrowck-use-uninitialized-in-cast-trait.stderr

@@ -8,8 +8,8 @@ LL |     let y = x as *const dyn Foo;
    |
 help: consider assigning a value
    |
-LL |     let x: &i32 = todo!();
-   |                 +++++++++
+LL |     let x: &i32 = value;
+   |                 +++++++
 
 error: aborting due to 1 previous error