fix for #139999

Author: Kivooeo

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -2772,6 +2772,32 @@ impl<'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
                         ),
                         _ => return None,
                     };
+
+                    // Heuristically determine whether a type name is likely intended to be a generic.
+                    //
+                    // We apply three rules:
+                    // 1. Short names (like `T`, `U`, `E`) are common for generics.
+                    // 2. Certain well-known names (e.g., `Item`, `Output`, `Error`) are commonly used as generics.
+                    // 3. Names in UpperCamelCase are more likely to be concrete types.
+                    //
+                    // This approach may produce false positives or negatives, but works well in most cases.
+
+                    let common_generic_names: &[&str] = &[
+                        "Item", "Output", "Error", "Target", "Value", "Args",
+                        "Res", "Ret", "This", "Iter", "Type",
+                    ];
+                    let is_common_generic = common_generic_names.contains(&sugg.as_str());
+                    let looks_like_camel_case = sugg
+                        .chars()
+                        .next()
+                        .map(|c| c.is_uppercase())
+                        .unwrap_or(false)
+                        && sugg.chars().skip(1).any(|c| c.is_lowercase());
+
+                    // If it's not a known generic name and looks like a concrete type, skip the suggestion.
+                    if !is_common_generic && looks_like_camel_case && sugg.len() > 3 {
+                        return None;
+                    }
                     let (span, sugg) = if let [.., param] = &generics.params[..] {
                         let span = if let [.., bound] = &param.bounds[..] {
                             bound.span()

tests/ui/consts/error-is-freeze.stderr

@@ -3,11 +3,6 @@ error[E0412]: cannot find type `UndefinedType` in this scope
    |
 LL |     foo: Option<UndefinedType>,
    |                 ^^^^^^^^^^^^^ not found in this scope
-   |
-help: you might be missing a type parameter
-   |
-LL | struct MyStruct<UndefinedType> {
-   |                +++++++++++++++
 
 error: aborting due to 1 previous error
 

tests/ui/issues/issue-58712.stderr

@@ -3,11 +3,6 @@ error[E0412]: cannot find type `DeviceId` in this scope
    |
 LL | impl<H> AddrVec<H, DeviceId> {
    |                    ^^^^^^^^ not found in this scope
-   |
-help: you might be missing a type parameter
-   |
-LL | impl<H, DeviceId> AddrVec<H, DeviceId> {
-   |       ++++++++++
 
 error[E0412]: cannot find type `DeviceId` in this scope
   --> $DIR/issue-58712.rs:8:29

tests/ui/layout/thaw-transmute-invalid-enum.stderr

@@ -3,11 +3,6 @@ error[E0412]: cannot find type `Subset` in this scope
    |
 LL |     assert::is_transmutable::<Superset, Subset>();
    |                                         ^^^^^^ not found in this scope
-   |
-help: you might be missing a type parameter
-   |
-LL | fn test<Subset>() {
-   |        ++++++++
 
 error[E0517]: attribute should be applied to a struct or union
   --> $DIR/thaw-transmute-invalid-enum.rs:21:11

tests/ui/pattern/usefulness/issue-119493-type-error-ice.stderr

@@ -10,10 +10,7 @@ error[E0412]: cannot find type `NonExistent` in this scope
 LL |     struct Foo(NonExistent);
    |                ^^^^^^^^^^^ not found in this scope
    |
-help: you might be missing a type parameter
-   |
-LL |     struct Foo<NonExistent>(NonExistent);
-   |               +++++++++++++
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 error[E0658]: `impl Trait` in type aliases is unstable
   --> $DIR/issue-119493-type-error-ice.rs:9:14

tests/ui/traits/issue-50480.stderr

@@ -32,10 +32,7 @@ error[E0412]: cannot find type `NotDefined` in this scope
 LL | struct Foo(N, NotDefined, <i32 as Iterator>::Item, Vec<i32>, String);
    |               ^^^^^^^^^^ not found in this scope
    |
-help: you might be missing a type parameter
-   |
-LL | struct Foo<NotDefined>(N, NotDefined, <i32 as Iterator>::Item, Vec<i32>, String);
-   |           ++++++++++++
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 error[E0412]: cannot find type `N` in this scope
   --> $DIR/issue-50480.rs:14:18

tests/ui/traits/missing-type-param-suggestion-issue-139999.rs

@@ -0,0 +1,6 @@
+fn do_something() -> Option<ForgotToImport> {
+    //~^ cannot find type `ForgotToImport` in this scope [E0412]
+    None
+}
+
+fn main() {}

tests/ui/traits/missing-type-param-suggestion-issue-139999.stderr

@@ -0,0 +1,9 @@
+error[E0412]: cannot find type `ForgotToImport` in this scope
+  --> $DIR/missing-type-param-suggestion-issue-139999.rs:1:29
+   |
+LL | fn do_something() -> Option<ForgotToImport> {
+   |                             ^^^^^^^^^^^^^^ not found in this scope
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0412`.

tests/ui/traits/trait-selection-ice-84727.stderr

@@ -21,11 +21,6 @@ error[E0412]: cannot find type `NewBg` in this scope
    |
 LL |     fn over(self) -> Cell<NewBg> {
    |                           ^^^^^ not found in this scope
-   |
-help: you might be missing a type parameter
-   |
-LL | impl<'b, TopFg, TopBg, BottomFg, BottomBg, NewBg> Over<&Cell<BottomFg, BottomBg>, ()>
-   |                                          +++++++
 
 error[E0308]: mismatched types
   --> $DIR/trait-selection-ice-84727.rs:21:22