Make E0284 generic argument suggestions more explicit by Muhtasim-Rasheed · Pull Request #154557 · rust-lang/rust

Muhtasim-Rasheed · 2026-03-29T17:45:13Z

Previously, when type annotations were missing for a function call, rust suggested: "consider specifying the generic argument". This PR improves the diagnostics:

If only one generic type is missing:
"consider specifying a concrete type for the generic type <T>" with the turbofish being "::<SomeConcreteType>"
If only one const generic is missing:
"consider specifying a const for the const generic <CONST>" with the turbofish being "::<SOME_CONST>"

Multiple missing generics still produce the original more general suggestion

rustbot · 2026-03-29T17:45:17Z

Some changes occurred in need_type_info.rs

cc @lcnr

rustbot · 2026-03-29T17:45:19Z

r? @chenyukang

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fmease · 2026-03-29T19:38:30Z

+    #[suggestion(
+        "consider specifying a concrete type for the generic type `{$arg}`",
+        style = "verbose",
+        code = "::<SomeConcreteType>",


The convention for placeholders nowadays is /* … */

Suggested change

code = "::<SomeConcreteType>",

code = "::</* Type */>",

fmease · 2026-03-29T19:38:50Z

+    #[suggestion(
+        "consider specifying a const for the const generic `{$arg}`",
+        style = "verbose",
+        code = "::<SOME_CONST>",


Suggested change

code = "::<SOME_CONST>",

code = "::</* CONST */>",

ditto

fmease · 2026-03-29T19:39:20Z

        args: String,
    },
+    #[suggestion(
+        "consider specifying a concrete type for the generic type `{$arg}`",


Suggested change

"consider specifying a concrete type for the generic type `{$arg}`",

"consider specifying a concrete type for the type parameter `{$param}`",

fmease · 2026-03-29T19:39:34Z

+        arg: String,
+    },
+    #[suggestion(
+        "consider specifying a const for the const generic `{$arg}`",


Suggested change

"consider specifying a const for the const generic `{$arg}`",

"consider specifying a value for the const parameter `{$param}`",

fmease · 2026-03-29T19:47:00Z

-                        args,
-                    });
+                    if generic_args.len() == 1
+                        && args.chars().all(|c| c.is_alphanumeric() || c == '_')


Looking through the generated text to heuristically figure out if this is meant to be a type or const param is iffy. After all args represents the list of arguments, not the list of (formal) parameters.

If you are set on suggesting a placeholder if the argument list "isn't meaningful" (for some sense of meaningful) I would look a few lines further up to decide that. Note that I haven't really looked at this PR in detail so I don't know the criteria you're looking for.

chenyukang · 2026-04-19T03:08:22Z

@rustbot author

rustbot · 2026-04-19T03:08:26Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

rustbot · 2026-04-20T10:48:49Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

Muhtasim-Rasheed · 2026-04-20T10:51:03Z

@rustbot ready

chenyukang · 2026-04-21T00:48:12Z

@bors r=chenyukang

rust-bors · 2026-04-21T00:48:15Z

📌 Commit b178681 has been approved by chenyukang

It is now in the queue for this repository.

Rollup of 5 pull requests Successful merges: - #153411 (Offload slice support) - #154557 (Make E0284 generic argument suggestions more explicit) - #154933 (Suggest removing `&` when awaiting a reference to a future) - #155524 (Fix LLVM offload install docs to use semicolon-separated CMake lists) - #155568 (Update books)

Rollup merge of #154557 - Muhtasim-Rasheed:issue-147313-improve-help, r=chenyukang Make E0284 generic argument suggestions more explicit Closes #147313 Previously, when type annotations were missing for a function call, rust suggested: "consider specifying the generic argument". This PR improves the diagnostics: - If only one generic type is missing: "consider specifying a concrete type for the generic type `<T>`" with the turbofish being "::\<SomeConcreteType>" - If only one const generic is missing: "consider specifying a const for the const generic `<CONST>`" with the turbofish being "::<SOME_CONST>" Multiple missing generics still produce the original more general suggestion

Rollup of 5 pull requests Successful merges: - rust-lang/rust#153411 (Offload slice support) - rust-lang/rust#154557 (Make E0284 generic argument suggestions more explicit) - rust-lang/rust#154933 (Suggest removing `&` when awaiting a reference to a future) - rust-lang/rust#155524 (Fix LLVM offload install docs to use semicolon-separated CMake lists) - rust-lang/rust#155568 (Update books)

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 Mar 29, 2026

rustbot assigned chenyukang Mar 29, 2026

fmease reviewed Mar 29, 2026

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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 19, 2026

Make E0284 generic argument suggestions more explicit

b178681

Muhtasim-Rasheed force-pushed the issue-147313-improve-help branch from f4f12fc to b178681 Compare April 20, 2026 10:48

rustbot added the WG-trait-system-refactor The Rustc Trait System Refactor Initiative (-Znext-solver) label Apr 20, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Apr 20, 2026

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Apr 21, 2026

jhpratt mentioned this pull request Apr 21, 2026

Rollup of 5 pull requests #155576

Merged

rust-bors Bot merged commit a8e1664 into rust-lang:main Apr 21, 2026
11 checks passed

rustbot added this to the 1.97.0 milestone Apr 21, 2026

	"consider specifying a concrete type for the generic type `{$arg}`",
	"consider specifying a concrete type for the type parameter `{$param}`",

	"consider specifying a const for the const generic `{$arg}`",
	"consider specifying a value for the const parameter `{$param}`",

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Muhtasim-Rasheed commented Mar 29, 2026 •

edited

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