Rollup merge of #100168 - WaffleLapkin:improve_diagnostics_for_missin…

…g_type_in_a_const_item, r=compiler-errors

Improve diagnostics for `const a: = expr;`

Adds a suggestion to write a type when there is a colon, but the type is not present.
I've also shrunk spans a little, so the suggestions are a little nicer.

Resolves #100146

r? `@compiler-errors`

compiler/rustc_parse/src/parser/item.rs

@@ -1179,39 +1179,47 @@ impl<'a> Parser<'a> {
 
         // Parse the type of a `const` or `static mut?` item.
         // That is, the `":" $ty` fragment.
-        let ty = if self.eat(&token::Colon) {
-            self.parse_ty()?
-        } else {
-            self.recover_missing_const_type(id, m)
+        let ty = match (self.eat(&token::Colon), self.check(&token::Eq) | self.check(&token::Semi))
+        {
+            // If there wasn't a `:` or the colon was followed by a `=` or `;` recover a missing type.
+            (true, false) => self.parse_ty()?,
+            (colon, _) => self.recover_missing_const_type(colon, m),
         };
 
         let expr = if self.eat(&token::Eq) { Some(self.parse_expr()?) } else { None };
         self.expect_semi()?;
         Ok((id, ty, expr))
     }
 
-    /// We were supposed to parse `:` but the `:` was missing.
+    /// We were supposed to parse `":" $ty` but the `:` or the type was missing.
     /// This means that the type is missing.
-    fn recover_missing_const_type(&mut self, id: Ident, m: Option<Mutability>) -> P<Ty> {
+    fn recover_missing_const_type(&mut self, colon_present: bool, m: Option<Mutability>) -> P<Ty> {
         // Construct the error and stash it away with the hope
         // that typeck will later enrich the error with a type.
         let kind = match m {
             Some(Mutability::Mut) => "static mut",
             Some(Mutability::Not) => "static",
             None => "const",
         };
-        let mut err = self.struct_span_err(id.span, &format!("missing type for `{kind}` item"));
+
+        let colon = match colon_present {
+            true => "",
+            false => ":",
+        };
+
+        let span = self.prev_token.span.shrink_to_hi();
+        let mut err = self.struct_span_err(span, &format!("missing type for `{kind}` item"));
         err.span_suggestion(
-            id.span,
+            span,
             "provide a type for the item",
-            format!("{id}: <type>"),
+            format!("{colon} <type>"),
             Applicability::HasPlaceholders,
         );
-        err.stash(id.span, StashKey::ItemNoType);
+        err.stash(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, tokens: None })
+        P(Ty { kind: TyKind::Infer, span, id: ast::DUMMY_NODE_ID, tokens: None })
     }
 
     /// Parses an enum declaration.

compiler/rustc_typeck/src/collect/type_of.rs

@@ -801,6 +801,9 @@ fn infer_placeholder_type<'a>(
     match tcx.sess.diagnostic().steal_diagnostic(span, StashKey::ItemNoType) {
         Some(mut err) => {
             if !ty.references_error() {
+                // Only suggest adding `:` if it was missing (and suggested by parsing diagnostic)
+                let colon = if span == item_ident.span.shrink_to_hi() { ":" } else { "" };
+
                 // The parser provided a sub-optimal `HasPlaceholders` suggestion for the type.
                 // We are typeck and have the real type, so remove that and suggest the actual type.
                 // FIXME(eddyb) this looks like it should be functionality on `Diagnostic`.
@@ -816,7 +819,7 @@ fn infer_placeholder_type<'a>(
                     err.span_suggestion(
                         span,
                         &format!("provide a type for the {item}", item = kind),
-                        format!("{}: {}", item_ident, sugg_ty),
+                        format!("{colon} {sugg_ty}"),
                         Applicability::MachineApplicable,
                     );
                 } else {

src/test/ui/attributes/issue-90873.stderr

@@ -34,10 +34,10 @@ LL | #![a={impl std::ops::Neg for i8 {}}]
    |                                     ^ consider adding a `main` function to `$DIR/issue-90873.rs`
 
 error: missing type for `static` item
-  --> $DIR/issue-90873.rs:1:16
+  --> $DIR/issue-90873.rs:1:17
    |
 LL | #![u=||{static d=||1;}]
-   |                ^ help: provide a type for the item: `d: <type>`
+   |                 ^ help: provide a type for the item: `: <type>`
 
 error: aborting due to 6 previous errors
 

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

@@ -17,10 +17,10 @@ LL | | }
    = note: this error originates in the macro `suite` (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
+  --> $DIR/issue-69396-const-no-type-in-macro.rs:4:20
    |
 LL |               const A = "A".$fn();
-   |                     ^ help: provide a type for the constant: `A: usize`
+   |                      ^ help: provide a type for the constant: `: usize`
 ...
 LL | / suite! {
 LL | |     len;
@@ -31,13 +31,13 @@ LL | | }
    = note: this error originates in the macro `suite` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0121]: the placeholder `_` is not allowed within types on item signatures for constants
-  --> $DIR/issue-69396-const-no-type-in-macro.rs:4:19
+  --> $DIR/issue-69396-const-no-type-in-macro.rs:4:20
    |
 LL |               const A = "A".$fn();
-   |                     ^
-   |                     |
-   |                     not allowed in type signatures
-   |                     help: replace with the correct type: `bool`
+   |                      ^
+   |                      |
+   |                      not allowed in type signatures
+   |                      help: replace with the correct type: `bool`
 ...
 LL | / suite! {
 LL | |     len;

src/test/ui/parser/issues/issue-89574.stderr

@@ -1,8 +1,8 @@
 error: missing type for `const` item
-  --> $DIR/issue-89574.rs:2:11
+  --> $DIR/issue-89574.rs:2:22
    |
 LL |     const EMPTY_ARRAY = [];
-   |           ^^^^^^^^^^^ help: provide a type for the item: `EMPTY_ARRAY: <type>`
+   |                      ^ help: provide a type for the item: `: <type>`
 
 error: aborting due to previous error
 

src/test/ui/parser/item-free-const-no-body-semantic-fail.stderr

@@ -15,10 +15,10 @@ LL | const B;
    |        help: provide a definition for the constant: `= <expr>;`
 
 error: missing type for `const` item
-  --> $DIR/item-free-const-no-body-semantic-fail.rs:6:7
+  --> $DIR/item-free-const-no-body-semantic-fail.rs:6:8
    |
 LL | const B;
-   |       ^ help: provide a type for the item: `B: <type>`
+   |        ^ help: provide a type for the item: `: <type>`
 
 error: aborting due to 3 previous errors
 

src/test/ui/parser/item-free-static-no-body-semantic-fail.stderr

@@ -31,16 +31,16 @@ LL | static mut D;
    |             help: provide a definition for the static: `= <expr>;`
 
 error: missing type for `static` item
-  --> $DIR/item-free-static-no-body-semantic-fail.rs:6:8
+  --> $DIR/item-free-static-no-body-semantic-fail.rs:6:9
    |
 LL | static B;
-   |        ^ help: provide a type for the item: `B: <type>`
+   |         ^ help: provide a type for the item: `: <type>`
 
 error: missing type for `static mut` item
-  --> $DIR/item-free-static-no-body-semantic-fail.rs:10:12
+  --> $DIR/item-free-static-no-body-semantic-fail.rs:10:13
    |
 LL | static mut D;
-   |            ^ help: provide a type for the item: `D: <type>`
+   |             ^ help: provide a type for the item: `: <type>`
 
 error: aborting due to 6 previous errors
 

src/test/ui/parser/removed-syntax-static-fn.stderr

@@ -16,10 +16,10 @@ LL | }
    | - the item list ends here
 
 error: missing type for `static` item
-  --> $DIR/removed-syntax-static-fn.rs:4:12
+  --> $DIR/removed-syntax-static-fn.rs:4:14
    |
 LL |     static fn f() {}
-   |            ^^ help: provide a type for the item: `r#fn: <type>`
+   |              ^ help: provide a type for the item: `: <type>`
 
 error: aborting due to 3 previous errors
 

src/test/ui/suggestions/const-no-type.rs

@@ -14,38 +14,38 @@ fn main() {}
 const C2 = 42;
 //~^ ERROR missing type for `const` item
 //~| HELP provide a type for the item
-//~| SUGGESTION C2: <type>
+//~| SUGGESTION : <type>
 
 #[cfg(FALSE)]
 static S2 = "abc";
 //~^ ERROR missing type for `static` item
 //~| HELP provide a type for the item
-//~| SUGGESTION S2: <type>
+//~| SUGGESTION : <type>
 
 #[cfg(FALSE)]
 static mut SM2 = "abc";
 //~^ ERROR missing type for `static mut` item
 //~| HELP provide a type for the item
-//~| SUGGESTION SM2: <type>
+//~| SUGGESTION : <type>
 
 // These will, so the diagnostics should be stolen by typeck:
 
 const C = 42;
 //~^ ERROR missing type for `const` item
 //~| HELP provide a type for the constant
-//~| SUGGESTION C: i32
+//~| SUGGESTION : i32
 
 const D = &&42;
 //~^ ERROR missing type for `const` item
 //~| HELP provide a type for the constant
-//~| SUGGESTION D: &&i32
+//~| SUGGESTION : &&i32
 
 static S = Vec::<String>::new();
 //~^ ERROR missing type for `static` item
 //~| HELP provide a type for the static variable
-//~| SUGGESTION S: Vec<String>
+//~| SUGGESTION : Vec<String>
 
 static mut SM = "abc";
 //~^ ERROR missing type for `static mut` item
 //~| HELP provide a type for the static variable
-//~| SUGGESTION &str
+//~| SUGGESTION : &str

src/test/ui/suggestions/const-no-type.stderr

@@ -1,44 +1,44 @@
 error: missing type for `const` item
-  --> $DIR/const-no-type.rs:33:7
+  --> $DIR/const-no-type.rs:33:8
    |
 LL | const C = 42;
-   |       ^ help: provide a type for the constant: `C: i32`
+   |        ^ help: provide a type for the constant: `: i32`
 
 error: missing type for `const` item
-  --> $DIR/const-no-type.rs:38:7
+  --> $DIR/const-no-type.rs:38:8
    |
 LL | const D = &&42;
-   |       ^ help: provide a type for the constant: `D: &&i32`
+   |        ^ help: provide a type for the constant: `: &&i32`
 
 error: missing type for `static` item
-  --> $DIR/const-no-type.rs:43:8
+  --> $DIR/const-no-type.rs:43:9
    |
 LL | static S = Vec::<String>::new();
-   |        ^ help: provide a type for the static variable: `S: Vec<String>`
+   |         ^ help: provide a type for the static variable: `: Vec<String>`
 
 error: missing type for `static mut` item
-  --> $DIR/const-no-type.rs:48:12
+  --> $DIR/const-no-type.rs:48:14
    |
 LL | static mut SM = "abc";
-   |            ^^ help: provide a type for the static variable: `SM: &str`
+   |              ^ help: provide a type for the static variable: `: &str`
 
 error: missing type for `const` item
-  --> $DIR/const-no-type.rs:14:7
+  --> $DIR/const-no-type.rs:14:9
    |
 LL | const C2 = 42;
-   |       ^^ help: provide a type for the item: `C2: <type>`
+   |         ^ help: provide a type for the item: `: <type>`
 
 error: missing type for `static` item
-  --> $DIR/const-no-type.rs:20:8
+  --> $DIR/const-no-type.rs:20:10
    |
 LL | static S2 = "abc";
-   |        ^^ help: provide a type for the item: `S2: <type>`
+   |          ^ help: provide a type for the item: `: <type>`
 
 error: missing type for `static mut` item
-  --> $DIR/const-no-type.rs:26:12
+  --> $DIR/const-no-type.rs:26:15
    |
 LL | static mut SM2 = "abc";
-   |            ^^^ help: provide a type for the item: `SM2: <type>`
+   |               ^ help: provide a type for the item: `: <type>`
 
 error: aborting due to 7 previous errors
 

src/test/ui/suggestions/unnamable-types.stderr

@@ -1,8 +1,8 @@
 error: missing type for `const` item
-  --> $DIR/unnamable-types.rs:6:7
+  --> $DIR/unnamable-types.rs:6:8
    |
 LL | const A = 5;
-   |       ^ help: provide a type for the constant: `A: i32`
+   |        ^ help: provide a type for the constant: `: i32`
 
 error[E0121]: the placeholder `_` is not allowed within types on item signatures for static variables
   --> $DIR/unnamable-types.rs:10:11
@@ -26,10 +26,10 @@ LL | const C: _ = || 42;
    |              ^^^^^
 
 error: missing type for `const` item
-  --> $DIR/unnamable-types.rs:23:7
+  --> $DIR/unnamable-types.rs:23:8
    |
 LL | const D = S { t: { let i = 0; move || -> i32 { i } } };
-   |       ^
+   |        ^
    |
 note: however, the inferred type `S<[closure@$DIR/unnamable-types.rs:23:31: 23:45]>` cannot be named
   --> $DIR/unnamable-types.rs:23:11
@@ -38,22 +38,22 @@ LL | const D = S { t: { let i = 0; move || -> i32 { i } } };
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: missing type for `const` item
-  --> $DIR/unnamable-types.rs:29:7
+  --> $DIR/unnamable-types.rs:29:8
    |
 LL | const E = foo;
-   |       ^ help: provide a type for the constant: `E: fn() -> i32`
+   |        ^ help: provide a type for the constant: `: fn() -> i32`
 
 error: missing type for `const` item
-  --> $DIR/unnamable-types.rs:32:7
+  --> $DIR/unnamable-types.rs:32:8
    |
 LL | const F = S { t: foo };
-   |       ^ help: provide a type for the constant: `F: S<fn() -> i32>`
+   |        ^ help: provide a type for the constant: `: S<fn() -> i32>`
 
 error: missing type for `const` item
-  --> $DIR/unnamable-types.rs:37:7
+  --> $DIR/unnamable-types.rs:37:8
    |
 LL | const G = || -> i32 { yield 0; return 1; };
-   |       ^
+   |        ^
    |
 note: however, the inferred type `[generator@$DIR/unnamable-types.rs:37:11: 37:20]` cannot be named
   --> $DIR/unnamable-types.rs:37:11

src/test/ui/typeck/issue-100164.fixed

@@ -0,0 +1,9 @@
+// run-rustfix
+
+const _A: i32 = 123;
+//~^ ERROR: missing type for `const` item
+
+fn main() {
+    const _B: i32 = 123;
+    //~^ ERROR: missing type for `const` item
+}

src/test/ui/typeck/issue-100164.rs

@@ -0,0 +1,9 @@
+// run-rustfix
+
+const _A: = 123;
+//~^ ERROR: missing type for `const` item
+
+fn main() {
+    const _B: = 123;
+    //~^ ERROR: missing type for `const` item
+}

src/test/ui/typeck/issue-100164.stderr

@@ -0,0 +1,14 @@
+error: missing type for `const` item
+  --> $DIR/issue-100164.rs:3:10
+   |
+LL | const _A: = 123;
+   |          ^ help: provide a type for the constant: `i32`
+
+error: missing type for `const` item
+  --> $DIR/issue-100164.rs:7:14
+   |
+LL |     const _B: = 123;
+   |              ^ help: provide a type for the constant: `i32`
+
+error: aborting due to 2 previous errors
+

src/test/ui/typeck/issue-79040.stderr

@@ -7,10 +7,10 @@ LL |     const FOO = "hello" + 1;
    |                 &str
 
 error: missing type for `const` item
-  --> $DIR/issue-79040.rs:2:11
+  --> $DIR/issue-79040.rs:2:14
    |
 LL |     const FOO = "hello" + 1;
-   |           ^^^ help: provide a type for the item: `FOO: <type>`
+   |              ^ help: provide a type for the item: `: <type>`
 
 error: aborting due to 2 previous errors
 

src/test/ui/typeck/typeck_type_placeholder_item.stderr

@@ -189,10 +189,10 @@ LL ~     b: (T, T),
    |
 
 error: missing type for `static` item
-  --> $DIR/typeck_type_placeholder_item.rs:73:12
+  --> $DIR/typeck_type_placeholder_item.rs:73:13
    |
 LL |     static A = 42;
-   |            ^ help: provide a type for the static variable: `A: i32`
+   |             ^ help: provide a type for the static variable: `: i32`
 
 error[E0121]: the placeholder `_` is not allowed within types on item signatures for static variables
   --> $DIR/typeck_type_placeholder_item.rs:75:15