Fix and improve diagnostics for lint `rust_2021_prefixes_incompatible_syntax` by fmease · Pull Request #161792 · rust-lang/rust

fmease · 2026-08-26T00:59:55Z

On main, when encountering token sequences in Rust <2021 that would get interpreted as...

...C string literals in Rust >=2021,
1. we report "prefix c is unknown" / "prefix cr is unknown" despite them obviously being known
2. we claim that they "[are] a hard error in Rust 2021"
...raw lifetimes in Rust >=2021 like 'r#a, we suggest splitting them after the hash (so 'r# and a) which obviously doesn't fix the issue; they need to be split before the hash (so 'r and #a)
...unknown (reserved) prefixes in Rust >=2021, we report "prefix […] is unknown" but to be pedantic the sequence is not a prefix in the current edition, it's just a normal identifier; to be correct & precise we should thus say that they will be parsed as a prefix (in Rust >=2021)

Lastly, unify / streamline all diagnostics pertaining to "reserved prefixes" (identifier/literal prefixes, hash prefixes (aka guarded string prefixes / multihashes))

Best reviewed commit by commit.

Note

Unblocks PR #161775.

_{(No LLM was or will be used by me during the entire creation process of this PR)}

rustbot · 2026-08-26T01:00:00Z

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rustbot · 2026-08-27T14:36:33Z

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.

mejrs

r? me

I agree with the general approach/direction here, just have some comments

View changes since this review

mejrs · 2026-08-30T16:35:14Z

+impl<'a> Diagnostic<'a, ()> for ReservedPrefixLint {
+    fn into_diag(self, dcx: DiagCtxtHandle<'a>, level: Level) -> Diag<'a, ()> {
+        let Self { kind, edition, sugg } = self;

-#[derive(Diagnostic)]
-#[diag("reserved token in Rust 2024")]
-pub(crate) struct ReservedMultihashLint {
-    #[suggestion(
-        "insert whitespace here to avoid this being parsed as a forbidden token in Rust 2024",
-        code = " ",
-        applicability = "machine-applicable"
-    )]
-    pub suggestion: Span,
+        Diag::new(dcx, level, format!("parsed as a {kind} in Rust {edition} and onward"))
+            .with_span_suggestion_verbose(
+                sugg,
+                "consider inserting whitespace here to avoid this",
+                " ",
+                Applicability::MachineApplicable,
+            )
+    }


Is this manual impl really necessary? You can use #[suggestion_verbose(..)] if it's just about the verbosity.

Ah, it's a remnant from when I had an enum w/ payloads for the kind and impl'ed it manually to avoid duplicating the mostly shared suggestion; now I should indeed be able to use the derive.

mejrs · 2026-08-30T16:37:47Z

@@ -1,69 +1,69 @@
-warning: prefix `z` is unknown
+warning: parsed as a prefix in Rust 2021 and onward


Can we change this lint to say what exactly the prefix would be?

Suggested change

warning: parsed as a prefix in Rust 2021 and onward

warning: `z` is parsed as a prefix in Rust 2021 and onward

mejrs · 2026-08-30T16:40:26Z

+                        span,
+                        ast::CRATE_NODE_ID,
+                        crate::diagnostics::ReservedPrefixLint {
+                            kind: "C string literal",


This could also be a "raw C string literal"

fmease · 2026-08-30T17:30:09Z

Applied all 3 suggestions.

mejrs

Thanks!

@bors r+ rollup

View changes since this review

rust-bors · 2026-08-30T18:01:55Z

📌 Commit 3fca755 has been approved by mejrs

It is now in the queue for this repository.

…diags, r=mejrs Fix and improve diagnostics for lint `rust_2021_prefixes_incompatible_syntax` On main, when encountering token sequences in Rust <2021 that would get interpreted as... 1. ...C string literals in Rust >=2021, 1. we report "*prefix `c` is unknown*" / "*prefix `cr` is unknown*" despite them obviously being known 2. we claim that they "*[are] a hard error in Rust 2021*" 2. ...raw lifetimes in Rust >=2021 like `'r#a`, we suggest splitting them *after* the hash (so `'r#` and `a`) which obviously doesn't fix the issue; they need to be split *before* the hash (so `'r` and `#a`) 3. ...unknown (reserved) prefixes in Rust >=2021, we report "*prefix […] is unknown*" but to be pedantic the sequence is *not* a prefix in the current edition, it's just a normal identifier; to be correct & precise we should thus say that they *will* be parsed as a prefix (in Rust >=2021) Lastly, unify / streamline all diagnostics pertaining to "reserved prefixes" (identifier/literal prefixes, hash prefixes (aka guarded string prefixes / multihashes)) --- Best reviewed commit by commit. > [!NOTE] > **Unblocks** PR rust-lang#161775. <sub>(No LLM was or will be used by me during the entire creation process of this PR)</sub>

…uwer Rollup of 11 pull requests Successful merges: - #160923 (Handle multiple action records in EH personality function) - #161788 (Check to ensure we're running against the correct LLVM version) - #158303 (Always escape grapheme extenders in `str::escape_debug`) - #161644 (bootstrap: Flatten and rename `compute_src_directory_via_git`) - #161702 (Use `drop_guard` in some places in {core,alloc,std}) - #161713 (Add config for gram editor to .gitignore) - #161792 (Fix and improve diagnostics for lint `rust_2021_prefixes_incompatible_syntax`) - #161931 (Revert "Add rustc_test_entrypoint_marker") - #162015 (touch up "get attribute" docs.) - #162019 (remove a couple of redundant clones, thanks clippy) - #162023 (Remove redundant braces from `NonZero` doctests)

Rollup merge of #161792 - fmease:fix-reserved-prefixes-lint-diags, r=mejrs Fix and improve diagnostics for lint `rust_2021_prefixes_incompatible_syntax` On main, when encountering token sequences in Rust <2021 that would get interpreted as... 1. ...C string literals in Rust >=2021, 1. we report "*prefix `c` is unknown*" / "*prefix `cr` is unknown*" despite them obviously being known 2. we claim that they "*[are] a hard error in Rust 2021*" 2. ...raw lifetimes in Rust >=2021 like `'r#a`, we suggest splitting them *after* the hash (so `'r#` and `a`) which obviously doesn't fix the issue; they need to be split *before* the hash (so `'r` and `#a`) 3. ...unknown (reserved) prefixes in Rust >=2021, we report "*prefix […] is unknown*" but to be pedantic the sequence is *not* a prefix in the current edition, it's just a normal identifier; to be correct & precise we should thus say that they *will* be parsed as a prefix (in Rust >=2021) Lastly, unify / streamline all diagnostics pertaining to "reserved prefixes" (identifier/literal prefixes, hash prefixes (aka guarded string prefixes / multihashes)) --- Best reviewed commit by commit. > [!NOTE] > **Unblocks** PR #161775. <sub>(No LLM was or will be used by me during the entire creation process of this PR)</sub>

rustbot assigned adwinwhite Aug 26, 2026

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 Aug 26, 2026

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fmease commented Aug 26, 2026

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Comment thread compiler/rustc_lint_defs/src/builtin.rs

fmease force-pushed the fix-reserved-prefixes-lint-diags branch from 4bb6b90 to cfc1f5f Compare August 26, 2026 01:09

Add test cases that demonstrate incorrect diagnostics

74ddcf2

fmease mentioned this pull request Aug 27, 2026

[STACKED] Implement forced keywords (k#) #161775

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fmease force-pushed the fix-reserved-prefixes-lint-diags branch from cfc1f5f to 1df00ae Compare August 27, 2026 14:36

mejrs requested changes Aug 30, 2026

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rustbot assigned mejrs and unassigned adwinwhite Aug 30, 2026

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 Aug 30, 2026

fmease force-pushed the fix-reserved-prefixes-lint-diags branch from 1df00ae to 3fca755 Compare August 30, 2026 17:29

fmease added 2 commits August 30, 2026 19:29

Unify and improve lint diagnostics for reserved prefixes

08f3b0e

Fix the suggestion span used for splitting raw lifetimes apart

3fca755

fmease 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 Aug 30, 2026

fmease requested a review from mejrs August 30, 2026 17:30

mejrs approved these changes Aug 30, 2026

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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 Aug 30, 2026

JonathanBrouwer mentioned this pull request Aug 30, 2026

Rollup of 11 pull requests #162024

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This was referenced Aug 30, 2026

Rollup of 11 pull requests #162025

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Rollup of 11 pull requests #162028

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rust-bors Bot merged commit dd9c329 into rust-lang:main Aug 31, 2026
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rustbot added this to the 1.100.0 milestone Aug 31, 2026

fmease deleted the fix-reserved-prefixes-lint-diags branch August 31, 2026 04:20

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Fix and improve diagnostics for lint `rust_2021_prefixes_incompatible_syntax` - #161792

Fix and improve diagnostics for lint `rust_2021_prefixes_incompatible_syntax`#161792
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		@@ -1,69 +1,69 @@
		warning: prefix `z` is unknown
		warning: parsed as a prefix in Rust 2021 and onward

	warning: parsed as a prefix in Rust 2021 and onward
	warning: `z` is parsed as a prefix in Rust 2021 and onward

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