Use correct span for structured suggestion

On structured suggestion for `let` -> `const`  and `const` -> `let`, use
a proper `Span` and update tests to check the correct application.

Follow up to #80012.

2 files changed, 31 insertions, 16 deletions

diff --git a/compiler/rustc_resolve/src/diagnostics.rs b/compiler/rustc_resolve/src/diagnostics.rs
index 6a181dbab5a..03b66a3f7b3 100644
--- a/compiler/rustc_resolve/src/diagnostics.rs
+++ b/compiler/rustc_resolve/src/diagnostics.rs
@@ -398,20 +398,30 @@ impl<'a> Resolver<'a> {
                 err.help("use the `|| { ... }` closure form instead");
                 err
             }
-            ResolutionError::AttemptToUseNonConstantValueInConstant(ident, sugg) => {
+            ResolutionError::AttemptToUseNonConstantValueInConstant(ident, sugg, current) => {
                 let mut err = struct_span_err!(
                     self.session,
                     span,
                     E0435,
                     "attempt to use a non-constant value in a constant"
                 );
-                err.span_suggestion(
-                    ident.span,
-                    &sugg,
-                    "".to_string(),
-                    Applicability::MaybeIncorrect,
-                );
-                err.span_label(span, "non-constant value");
+                // let foo =...
+                //     ^^^ given this Span
+                // ------- get this Span to have an applicable suggestion
+                let sp =
+                    self.session.source_map().span_extend_to_prev_str(ident.span, current, true);
+                if sp.lo().0 == 0 {
+                    err.span_label(ident.span, &format!("this would need to be a `{}`", sugg));
+                } else {
+                    let sp = sp.with_lo(BytePos(sp.lo().0 - current.len() as u32));
+                    err.span_suggestion(
+                        sp,
+                        &format!("consider using `{}` instead of `{}`", sugg, current),
+                        format!("{} {}", sugg, ident),
+                        Applicability::MaybeIncorrect,
+                    );
+                    err.span_label(span, "non-constant value");
+                }
                 err
             }
             ResolutionError::BindingShadowsSomethingUnacceptable(what_binding, name, binding) => {
diff --git a/compiler/rustc_resolve/src/lib.rs b/compiler/rustc_resolve/src/lib.rs
index a6d0240b6fd..2c68e041858 100644
--- a/compiler/rustc_resolve/src/lib.rs
+++ b/compiler/rustc_resolve/src/lib.rs
@@ -210,7 +210,11 @@ enum ResolutionError<'a> {
     /// Error E0434: can't capture dynamic environment in a fn item.
     CannotCaptureDynamicEnvironmentInFnItem,
     /// Error E0435: attempt to use a non-constant value in a constant.
-    AttemptToUseNonConstantValueInConstant(Ident, String),
+    AttemptToUseNonConstantValueInConstant(
+        Ident,
+        /* suggestion */ &'static str,
+        /* current */ &'static str,
+    ),
     /// Error E0530: `X` bindings cannot shadow `Y`s.
     BindingShadowsSomethingUnacceptable(&'static str, Symbol, &'a NameBinding<'a>),
     /// Error E0128: type parameters with a default cannot use forward-declared identifiers.
@@ -2614,18 +2618,19 @@ impl<'a> Resolver<'a> {
                                             ConstantItemKind::Const => "const",
                                             ConstantItemKind::Static => "static",
                                         };
-                                        let sugg = format!(
-                                            "consider using `let` instead of `{}`",
-                                            kind_str
-                                        );
-                                        (span, AttemptToUseNonConstantValueInConstant(ident, sugg))
+                                        (
+                                            span,
+                                            AttemptToUseNonConstantValueInConstant(
+                                                ident, "let", kind_str,
+                                            ),
+                                        )
                                     } else {
-                                        let sugg = "consider using `const` instead of `let`";
                                         (
                                             rib_ident.span,
                                             AttemptToUseNonConstantValueInConstant(
                                                 original_rib_ident_def,
-                                                sugg.to_string(),
+                                                "const",
+                                                "let",
                                             ),
                                         )
                                     };