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Improve type parameter suggestion heuristic for missing types #140073

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27 changes: 26 additions & 1 deletion compiler/rustc_resolve/src/late/diagnostics.rs
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Expand Up @@ -2764,14 +2764,39 @@ impl<'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {

let (msg, sugg) = match source {
PathSource::Type | PathSource::PreciseCapturingArg(TypeNS) => {
("you might be missing a type parameter", ident)
("you might be missing a type parameter", ident.clone())
}
PathSource::Expr(_) | PathSource::PreciseCapturingArg(ValueNS) => (
"you might be missing a const parameter",
format!("const {ident}: /* Type */"),
),
_ => 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] = &[
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Suggested change
let common_generic_names: &[&str] = &[
let common_type_param_names: &[&str] = &[

since it doesn't make sense for const params

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Also, I'd like to draw attention to the

    && segment.ident.name != kw::SelfUpper
    && segment.ident.name != kw::Dyn =>

at the very start of the function. We're already filtering out certain identifiers (in this case Self and dyn). I'd like us to unify this check and the common_generic_names one is some way or another

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@Kivooeo Kivooeo Apr 22, 2025 •
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this is good point, will it be better to move my logic right after this let (ident, span) = ... or integrate them inside the match?

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I think what @fmease might expect is a change like...

        let [segment] = segments else {
               return None;
         };

          // comment here explaining why we skip these
         let skip_names = [kw::SelfUpper, kw::Dyn, sym::This, sym::Error, /* etc */];

         if !segment.has_generic_args && !skip_names.contains(segment.ident.name) {
                 return None;
         }
         let ident = segment.ident.to_string();
         let span =  segment.ident.span;

I personally much prefer let_else over a match in this case. Feel free to write it with a match if you prefer.

Also, not all these names are available as symbols in sym. Feel free to add them there as needed,

"Item", "Output", "Error", "Target", "Value", "Args",
"Res", "Ret", "This", "Iter", "Type",
];
let is_common_generic = common_generic_names.contains(&&*ident);
let looks_like_camel_case = ident
.chars()
.next()
.is_some_and(|c| c.is_uppercase())
&& ident.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 && ident.len() > 3 {
return None;
}
let (span, sugg) = if let [.., param] = &generics.params[..] {
let span = if let [.., bound] = &param.bounds[..] {
bound.span()
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5 changes: 0 additions & 5 deletions tests/ui/const-generics/generic_const_exprs/issue-109141.stderr
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Expand Up @@ -3,11 +3,6 @@ error[E0425]: cannot find value `HashesEntryLEN` in this scope
|
LL | struct EntriesBuffer(Box<[[u8; HashesEntryLEN]; 5]>);
| ^^^^^^^^^^^^^^ not found in this scope
|
help: you might be missing a const parameter
|
LL | struct EntriesBuffer<const HashesEntryLEN: /* Type */>(Box<[[u8; HashesEntryLEN]; 5]>);
| ++++++++++++++++++++++++++++++++++

error: aborting due to 1 previous error

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5 changes: 0 additions & 5 deletions tests/ui/consts/error-is-freeze.stderr
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Expand Up @@ -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

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5 changes: 0 additions & 5 deletions tests/ui/issues/issue-58712.stderr
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Expand Up @@ -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
Expand Down
5 changes: 0 additions & 5 deletions tests/ui/layout/thaw-transmute-invalid-enum.stderr
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Expand Up @@ -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
Expand Down
17 changes: 16 additions & 1 deletion tests/ui/missing/missing-items/missing-type-parameter.rs
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@@ -1,4 +1,19 @@
fn foo<X>() { }
fn foo<X>() {}

fn do_something() -> Option<ForgotToImport> {
//~^ cannot find type `ForgotToImport` in this scope [E0412]
None
}

fn do_something_T() -> Option<T> {
//~^ cannot find type `T` in this scope [E0412]
None
}

fn do_something_Type() -> Option<Type> {
//~^ cannot find type `Type` in this scope [E0412]
None
}

fn main() {
foo(); //~ ERROR type annotations needed
Expand Down
35 changes: 32 additions & 3 deletions tests/ui/missing/missing-items/missing-type-parameter.stderr
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@@ -1,5 +1,33 @@
error[E0412]: cannot find type `ForgotToImport` in this scope
--> $DIR/missing-type-parameter.rs:3:29
|
LL | fn do_something() -> Option<ForgotToImport> {
| ^^^^^^^^^^^^^^ not found in this scope

error[E0412]: cannot find type `T` in this scope
--> $DIR/missing-type-parameter.rs:8:31
|
LL | fn do_something_T() -> Option<T> {
| ^ not found in this scope
|
help: you might be missing a type parameter
|
LL | fn do_something_T<T>() -> Option<T> {
| +++

error[E0412]: cannot find type `Type` in this scope
--> $DIR/missing-type-parameter.rs:13:34
|
LL | fn do_something_Type() -> Option<Type> {
| ^^^^ not found in this scope
|
help: you might be missing a type parameter
|
LL | fn do_something_Type<Type>() -> Option<Type> {
| ++++++

error[E0282]: type annotations needed
--> $DIR/missing-type-parameter.rs:4:5
--> $DIR/missing-type-parameter.rs:19:5
|
LL | foo();
| ^^^ cannot infer type of the type parameter `X` declared on the function `foo`
Expand All @@ -9,6 +37,7 @@ help: consider specifying the generic argument
LL | foo::<X>();
| +++++

error: aborting due to 1 previous error
error: aborting due to 4 previous errors

For more information about this error, try `rustc --explain E0282`.
Some errors have detailed explanations: E0282, E0412.
For more information about an error, try `rustc --explain E0282`.
5 changes: 1 addition & 4 deletions tests/ui/pattern/usefulness/issue-119493-type-error-ice.stderr
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Expand Up @@ -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
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5 changes: 1 addition & 4 deletions tests/ui/traits/issue-50480.stderr
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Expand Up @@ -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
Expand Down
5 changes: 0 additions & 5 deletions tests/ui/traits/trait-selection-ice-84727.stderr
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Expand Up @@ -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
Expand Down
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