make .stash(..) work in macros without ICEing

Author: Centril

src/librustc_errors/diagnostic_builder.rs

@@ -115,13 +115,28 @@ impl<'a> DiagnosticBuilder<'a> {
 
     /// Stashes diagnostic for possible later improvement in a different,
     /// later stage of the compiler. The diagnostic can be accessed with
-    /// the provided `span` and `key` through `.steal_diagnostic` on `Handler`.
+    /// the *returned* `span` and `key` through `.steal_diagnostic` on `Handler`.
+    /// Do not use the `span` passed into this function when calling `.steal_diagnostic`.
     ///
     /// As with `buffer`, this is unless the handler has disabled such buffering.
-    pub fn stash(self, span: Span, key: StashKey) {
-        if let Some((diag, handler)) = self.into_diagnostic() {
+    #[must_use = "`Span` returned by `.stash(...)` must be later used in `.steal_diagnostic(..)`"]
+    pub fn stash(self, span: Span, key: StashKey) -> Span {
+        self.into_diagnostic().map_or(span, |(diag, handler)| {
+            // Before stashing the diagnostic, make sure the `span` is unique so that
+            // we do not attempt overwriting the key's slot later when dealing with macros.
+            // If we don't do this, then something like the following...:
+            // ```
+            // macro_rules! suite {
+            //     ( $( $fn:ident; )* ) => { $(const A = "A".$fn();)* }
+            // }
+            //
+            // suite! { len; is_empty; }
+            // ```
+            // ...would result in overwriting due to using the same `def_site` span for `A`.
+            let span = span.fresh_expansion(span.ctxt().outer_expn_data());
             handler.stash_diagnostic(span, key, diag);
-        }
+            span
+        })
     }
 
     /// Converts the builder to a `Diagnostic` for later emission,

src/librustc_parse/parser/item.rs

@@ -936,11 +936,11 @@ impl<'a> Parser<'a> {
             format!("{}: <type>", id),
             Applicability::HasPlaceholders,
         );
-        err.stash(id.span, StashKey::ItemNoType);
+        let span = err.stash(id.span, StashKey::ItemNoType);
 
         // The user intended that the type be inferred,
         // so treat this as if the user wrote e.g. `const A: _ = expr;`.
-        P(Ty { kind: TyKind::Infer, span: id.span, id: ast::DUMMY_NODE_ID })
+        P(Ty { kind: TyKind::Infer, span, id: ast::DUMMY_NODE_ID })
     }
 
     /// Parses an enum declaration.

src/test/ui/issues/issue-69396-const-no-type-in-macro.rs

@@ -0,0 +1,17 @@
+macro_rules! suite {
+    ( $( $fn:ident; )* ) => {
+        $(
+            const A = "A".$fn();
+            //~^ ERROR the name `A` is defined multiple times
+            //~| ERROR missing type for `const` item
+            //~| ERROR missing type for `const` item
+        )*
+    }
+}
+
+suite! {
+    len;
+    is_empty;
+}
+
+fn main() {}

src/test/ui/issues/issue-69396-const-no-type-in-macro.stderr

@@ -0,0 +1,49 @@
+error[E0428]: the name `A` is defined multiple times
+  --> $DIR/issue-69396-const-no-type-in-macro.rs:4:13
+   |
+LL |               const A = "A".$fn();
+   |               ^^^^^^^^^^^^^^^^^^^^
+   |               |
+   |               `A` redefined here
+   |               previous definition of the value `A` here
+...
+LL | / suite! {
+LL | |     len;
+LL | |     is_empty;
+LL | | }
+   | |_- in this macro invocation
+   |
+   = note: `A` must be defined only once in the value namespace of this module
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: missing type for `const` item
+  --> $DIR/issue-69396-const-no-type-in-macro.rs:4:19
+   |
+LL |               const A = "A".$fn();
+   |                     ^ help: provide a type for the item: `A: usize`
+...
+LL | / suite! {
+LL | |     len;
+LL | |     is_empty;
+LL | | }
+   | |_- in this macro invocation
+   |
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: missing type for `const` item
+  --> $DIR/issue-69396-const-no-type-in-macro.rs:4:19
+   |
+LL |               const A = "A".$fn();
+   |                     ^ help: provide a type for the item: `A: bool`
+...
+LL | / suite! {
+LL | |     len;
+LL | |     is_empty;
+LL | | }
+   | |_- in this macro invocation
+   |
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0428`.