mbe: Defer checks for `compile_error!` until reporting an unused macro rule #143416

joshtriplett · 2025-07-04T08:50:39Z

The current MBE parser checks rules at initial parse time to see if their RHS has compile_error! in it, and returns a list of rule indexes and LHS spans that don't map to compile_error!, for use in unused macro rule checking.

Instead, have the unused macro rule reporting ask the macro for the rule to report, and let the macro check at that time. That avoids checking rules unless they're unused.

In the process, refactor the data structure used to store macro rules, to group the LHS and RHS (and LHS span) of each rule together, and refactor the unused rule tracking to only track rule indexes.

This builds atop a couple of minor MBE refactors. I would suggest reviewing commit-by-commit.

The overall result is a further simplification of the macro code.

rustbot · 2025-07-04T08:50:43Z

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joshtriplett · 2025-07-04T08:52:19Z

As a potential added improvement, it might make sense to change unused_macro_rules to use DenseBitSet in place of UnordSet. That'd be a slightly larger change, and I didn't want to mix it into this PR, but it seems worth trying. I'd like to get the perf numbers with and without that, though.

mbe: Defer checks for `compile_error!` until reporting an unused macro rule The current MBE parser checks rules at initial parse time to see if their RHS has `compile_error!` in it, and returns a list of rule indexes and LHS spans that don't map to `compile_error!`, for use in unused macro rule checking. Instead, have the unused macro rule reporting ask the macro for the rule to report, and let the macro check at that time. That avoids checking rules unless they're unused. In the process, refactor the data structure used to store macro rules, to group the LHS and RHS (and LHS span) of each rule together, and refactor the unused rule tracking to only track rule indexes. This builds atop a couple of minor MBE refactors. I would suggest reviewing commit-by-commit. This PR builds atop the fix for #143408 to avoid conflicts.

joshtriplett · 2025-07-04T08:59:12Z

@bors try @rust-timer queue

mbe: Defer checks for `compile_error!` until reporting an unused macro rule The current MBE parser checks rules at initial parse time to see if their RHS has `compile_error!` in it, and returns a list of rule indexes and LHS spans that don't map to `compile_error!`, for use in unused macro rule checking. Instead, have the unused macro rule reporting ask the macro for the rule to report, and let the macro check at that time. That avoids checking rules unless they're unused. In the process, refactor the data structure used to store macro rules, to group the LHS and RHS (and LHS span) of each rule together, and refactor the unused rule tracking to only track rule indexes. This builds atop a couple of minor MBE refactors. I would suggest reviewing commit-by-commit. This PR builds atop the fix for #143408 to avoid conflicts.

bors · 2025-07-04T09:00:07Z

⌛ Trying commit d6422f8 with merge 0d199d4...

bors · 2025-07-04T11:16:47Z

☀️ Try build successful - checks-actions
Build commit: 0d199d4 (0d199d444a215e6b14a6d0142770f95a1f5067eb)

rust-timer · 2025-07-04T16:25:54Z

Finished benchmarking commit (0d199d4): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.3%	[-0.3%, -0.3%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (secondary -6.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-6.0%	[-6.0%, -6.0%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary -4.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.7%	[0.6%, 2.4%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-9.0%	[-12.0%, -2.2%]	4
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 461.827s -> 462.538s (0.15%)
Artifact size: 372.20 MiB -> 372.20 MiB (-0.00%)

joshtriplett · 2025-07-04T17:25:30Z

@bors rollup

mbe: Change `unused_macro_rules` to a `DenseBitSet` Now that it only contains indexes, and no other information, a bitset provides a more compact and simpler representation. This builds on <#143416>. Only the last commit is new.

Rather than a `bool` that's `true` for the LHS and `false` for the RHS, use a self-documenting enum.

compiler/rustc_expand/src/mbe/quoted.rs

The parser repeatedly invokes the `parse` function, constructing a one-entry vector, and assuming that the return value will be a one-entry vector. Add a helper for that case. This will simplify adding additional callers, and put all the logic in one place to allow potential future simplification of the one-TT case.

…o rule The MBE parser checks rules at initial parse time to see if their RHS has `compile_error!` in it, and returns a list of rule indexes and LHS spans that don't map to `compile_error!`, for use in unused macro rule checking. Instead, have the unused macro rule reporting ask the macro for the rule to report, and let the macro check at that time. That avoids checking rules unless they're unused. In the process, refactor the data structure used to store macro rules, to group the LHS and RHS (and LHS span) of each rule together, and refactor the unused rule tracking to only track rule indexes. This ends up being a net simplification, and reduction in code size.

nnethercote · 2025-07-06T04:08:58Z

@bors r+

bors · 2025-07-06T04:09:00Z

📌 Commit 63cfb3a has been approved by nnethercote

It is now in the queue for this repository.

Rollup of 6 pull requests Successful merges: - #143416 (mbe: Defer checks for `compile_error!` until reporting an unused macro rule) - #143470 (std: sys: net: uefi: tcp4: Implement read) - #143477 (use `is_multiple_of` and `div_ceil`) - #143484 (distinguish the duplicate item of rpitit) - #143493 (tidy: use --bless for tidy spellcheck instead of spellcheck:fix) - #143504 (compiletest: print slightly more information on fs::write failure) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #143416 - joshtriplett:mbe-simplifications, r=nnethercote mbe: Defer checks for `compile_error!` until reporting an unused macro rule The current MBE parser checks rules at initial parse time to see if their RHS has `compile_error!` in it, and returns a list of rule indexes and LHS spans that don't map to `compile_error!`, for use in unused macro rule checking. Instead, have the unused macro rule reporting ask the macro for the rule to report, and let the macro check at that time. That avoids checking rules unless they're unused. In the process, refactor the data structure used to store macro rules, to group the LHS and RHS (and LHS span) of each rule together, and refactor the unused rule tracking to only track rule indexes. This builds atop a couple of minor MBE refactors. I would suggest reviewing commit-by-commit. The overall result is a further simplification of the macro code.

…et, r=lqd mbe: Change `unused_macro_rules` to a `DenseBitSet` Now that it only contains indexes, and no other information, a bitset provides a more compact and simpler representation. This builds on <rust-lang#143416>. Only the last commit is new.

Rollup merge of #143456 - joshtriplett:mbe-unused-rules-bitset, r=lqd mbe: Change `unused_macro_rules` to a `DenseBitSet` Now that it only contains indexes, and no other information, a bitset provides a more compact and simpler representation. This builds on <#143416>. Only the last commit is new.

petrochenkov · 2025-07-08T18:25:30Z

compiler/rustc_expand/src/base.rs

@@ -348,6 +348,10 @@ pub trait TTMacroExpander {
        span: Span,
        input: TokenStream,
    ) -> MacroExpanderResult<'cx>;
+
+    fn get_unused_rule(&self, _rule_i: usize) -> Option<(&Ident, Span)> {


This trait isn't supposed to know anything about rules or other ways to implement a macro, only about the macro's expander function.

It would probably be better to downcast from dyn TTMacroExpander (+ Any) to MacroRulesMacroExpander in check_unused_macros and extract the necessary data directly if the downcast succeeds.

Or just revert the change, it doesn't actually bring perf benefits, but complicates the logic.

joshtriplett added the A-macros Area: All kinds of macros (custom derive, macro_rules!, proc macros, ..) label Jul 4, 2025

rustbot assigned compiler-errors Jul 4, 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 Jul 4, 2025

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joshtriplett force-pushed the mbe-simplifications branch from 9c9bed5 to d6422f8 Compare July 4, 2025 08:58

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mbe: Change unused_macro_rules to a DenseBitSet #143456

Merged

mbe: Introduce an enum for which part of a rule we're parsing

6b71399

Rather than a `bool` that's `true` for the LHS and `false` for the RHS, use a self-documenting enum.

joshtriplett force-pushed the mbe-simplifications branch from d6422f8 to 3c1559c Compare July 5, 2025 10:07

nnethercote reviewed Jul 5, 2025

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compiler/rustc_expand/src/mbe/quoted.rs Outdated Show resolved Hide resolved

joshtriplett added 2 commits July 5, 2025 16:23

mbe: Simplify a match to a let-else

0d5ab3e

joshtriplett force-pushed the mbe-simplifications branch from 3c1559c to 63cfb3a Compare July 5, 2025 23:23

bors 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 Jul 6, 2025

matthiaskrgr mentioned this pull request Jul 6, 2025

Rollup of 6 pull requests #143521

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bors merged commit 71b73e5 into rust-lang:master Jul 6, 2025
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joshtriplett deleted the mbe-simplifications branch July 6, 2025 18:55

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mbe: Defer checks for `compile_error!` until reporting an unused macro rule #143416

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Overall result: ✅ improvements - no action needed

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mbe: Defer checks for `compile_error!` until reporting an unused macro rule #143416

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