Improve type parameter suggestion heuristic for missing types #140073

Kivooeo · 2025-04-20T11:18:16Z

I consider this solution a starting point and not final. I'm open to your ideas and suggestions for further improvements to the heuristic or implementation.

Summary

This PR improves the heuristic used by rustc to suggest type parameters for missing types, addressing issue #139999. The issue occurs when rustc mistakenly suggests adding a type parameter for a missing type (e.g., ForgotToImport) that is likely a concrete type rather than a generic. This can lead to confusing error messages, as shown in the example:

pub fn do_stuff() -> Option<ForgotToImport> {
    None
}

Current output suggests adding a type parameter:

help: you might be missing a type parameter
  |
1 | pub fn do_stuff<ForgotToImport>() -> Option<ForgotToImport> {
  |                ++++++++++++++++

This PR refines the heuristic to avoid such suggestions for names that are likely concrete types.

Changes

The fix modifies the logic in the type suggestion code to skip type parameter suggestions when the missing type name:

Is not a common generic name (e.g., Item, Output, Error).
Uses UpperCamelCase, which is typical for concrete types.
Is longer than 3 characters, as type parameters are often short (e.g., T, U).

Example

With this change, the example code above will no longer suggest a type parameter for ForgotToImport, resulting in a cleaner error message:

error[E0412]: cannot find type `ForgotToImport` in this scope
 --> src/lib.rs:1:29
  |
1 | pub fn do_stuff() -> Option<ForgotToImport> {
  |                             ^^^^^^^^^^^^^^ not found in this scope

Rationale

This approach balances precision and simplicity:

UpperCamelCase is a strong indicator of concrete types in Rust, as type parameters are often single letters or specific generic names.
The length check (>3) reduces false positives for longer names, which are less common for generics.
Preserving suggestions for well-known generic names ensures backward compatibility and avoids breaking valid cases.

Related Issue

fixes #139999

cc @Wilfred (issue reporter)

rustbot · 2025-04-20T11:18:21Z

r? @oli-obk

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oli-obk · 2025-04-22T11:18:04Z

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

Also test that a common name like Item still suggest making it generic

oli-obk · 2025-04-22T11:19:43Z

compiler/rustc_resolve/src/late/diagnostics.rs

+                        && 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 {


the len 3 check is always triggered on const parameter suggestions. check the length of ident instead. Also add a test for const generics, too

can you please show me example of test for const, because i dont see how can i do it

pub fn do_stuff<const T: usize>() -> Option<T> { None }

this surely wrong because trigger another errror
and

pub fn do_stuff<const T: usize>() -> Option<ForgetType> { None }

this works fine

and i cant do something like this because it's basically wrong

pub fn do_stuff() -> Option<const T: usize> { None }

so i need an example of how test it correctly

or do you meant something like this?

pub fn do_stuff() -> Option<MAX> { None }

MAX here looks like constant so should be a generic right?

fn f() -> [(); N] {}

this one works just fine i guess

error[E0425]: cannot find value `N` in this scope --> C:\Users\Love\projects\asfgas\src/main.rs:1:16 | 1 | fn f() -> [(); N] {} | ^ not found in this scope | help: you might be missing a const parameter | 1 | fn f<const N: /* Type */>() -> [(); N] {} | +++++++++++++++++++++

Oli is saying that for const parameters you currently perform checks like common_generic_names.contains("const N: usize") which will never fail! You should compare against the original ident (in this case N)

Note however that for const parameters, the list common_generic_names doesn't make sense, it's specifically for type parameters, so you should be comparing against a different list or none at all

yes i got it, but i was wondering how do i write tests for this const case more correctly, and i also want to ask if name is full uppercase, should we consider it as generic?

We should either use a different list for const params (of common const parameter names of length>1 (idk which ones could be considered common (LEN? LENGTH?, SIZE? perhaps I dunno)) or don't do such a check for them at all

and i also want to ask if name is full uppercase, should we consider it as generic?

I guess not necessarily, similar to (generic) type parameters. Since ELEMS can just as well refer to a const item. E.g.:

const ELEMS: u32 = 3; fn f() -> [i32; ELEMZ] { todo!() }

oli-obk · 2025-04-22T11:21:35Z

compiler/rustc_resolve/src/late/diagnostics.rs

+                        .map(|c| c.is_uppercase())
+                        .unwrap_or(false)


Suggested change

.map(|c| c.is_uppercase())

.unwrap_or(false)

.is_some_and(|c| c.is_uppercase())

oli-obk · 2025-04-22T11:22:45Z

To close the issue when this PR is merged, change your main comment to contain fixes #123456 (also please remove that from the title

fmease · 2025-04-22T12:15:08Z

compiler/rustc_resolve/src/late/diagnostics.rs

+                    //
+                    // This approach may produce false positives or negatives, but works well in most cases.
+
+                    let common_generic_names: &[&str] = &[


Suggested change

let common_generic_names: &[&str] = &[

let common_type_param_names: &[&str] = &[

since it doesn't make sense for const params

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

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

fmease · 2025-04-22T14:18:25Z

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

We already have a bunch of test files for this diagnostic:

tests/ui/missing/missing-items/missing-const-parameter.rs

tests/ui/missing/missing-items/missing-type-parameter2.rs

tests/ui/missing/missing-items/missing-type-parameter.rs

Could you merge your new test cases into the appropriate preexisting test files? That would great, thanks!

fmease · 2025-04-22T14:18:38Z

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

We already have a bunch of test files for this diagnostic:

tests/ui/missing/missing-items/missing-const-parameter.rs

tests/ui/missing/missing-items/missing-type-parameter2.rs

tests/ui/missing/missing-items/missing-type-parameter.rs

Could you merge your new test cases into the appropriate preexisting test files? That would great, thanks!

Kivooeo · 2025-04-22T14:20:02Z

@oli-obk @fmease check this changes, replaced sugg with ident and added more test cases.
i played around this check at function start and i didnt liked it much because split condition leads to not all cases are covered, and moving entire logic on top is worse because the place it current located filtering out main which is good for more precise error messaging.

and i also didnt liked idea about renaming array of common names, because it's just common generic names that looks like UpperCamelCase and it's len > 3, so this array and check is needed to remove false positive results

rustbot assigned oli-obk Apr 20, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Apr 20, 2025

Kivooeo force-pushed the fix-139999-type-suggestion branch from 95b39e7 to 037c08e Compare April 20, 2025 11:19

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Kivooeo force-pushed the fix-139999-type-suggestion branch from 660a5f4 to 9a92054 Compare April 20, 2025 11:35

oli-obk requested changes Apr 22, 2025

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rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Apr 22, 2025

Kivooeo changed the title ~~Improve type parameter suggestion heuristic for missing types (fixes #139999)~~ Improve type parameter suggestion heuristic for missing types Apr 22, 2025

fmease reviewed Apr 22, 2025

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fix for rust-lang#139999

e94d192

Kivooeo force-pushed the fix-139999-type-suggestion branch from 25300e3 to e94d192 Compare April 22, 2025 14:53

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Improve type parameter suggestion heuristic for missing types #140073

Improve type parameter suggestion heuristic for missing types #140073

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	.map(\|c\| c.is_uppercase())
	.unwrap_or(false)
	.is_some_and(\|c\| c.is_uppercase())

	let common_generic_names: &[&str] = &[
	let common_type_param_names: &[&str] = &[

Improve type parameter suggestion heuristic for missing types #140073

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Improve type parameter suggestion heuristic for missing types #140073

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Kivooeo commented Apr 20, 2025 • edited Loading

Summary

Changes

Example

Rationale

Related Issue

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