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macro_rules! macros do not backtrack #42838

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Mark-Simulacrum opened this issue Jun 22, 2017 · 2 comments
Open

macro_rules! macros do not backtrack #42838

Mark-Simulacrum opened this issue Jun 22, 2017 · 2 comments
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A-macros Area: All kinds of macros (custom derive, macro_rules!, proc macros, ..) C-feature-request Category: A feature request, i.e: not implemented / a PR.

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@Mark-Simulacrum
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I'm mainly filing this to be able to close other issues whose problems come down to this. I don't know if we want to fix this or will fix this, but at least there will be a single issue for it.
@Mark-Simulacrum Mark-Simulacrum added the A-macros Area: All kinds of macros (custom derive, macro_rules!, proc macros, ..) label Jun 22, 2017
@Mark-Simulacrum Mark-Simulacrum mentioned this issue Jun 22, 2017
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This was referenced Jun 26, 2017
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bors added a commit that referenced this issue Jul 11, 2017
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Auto merge of #42913 - kennytm:fix-40569-ident-without-backtrack, r=j…
1999bfa 
…seyfried

Only match a fragment specifier the if it starts with certain tokens.

When trying to match a fragment specifier, we first predict whether the current token can be matched at all. If it cannot be matched, don't bother to push the Earley item to `bb_eis`. This can fix a lot of issues which otherwise requires full backtracking (#42838).

In this PR the prediction treatment is not done for `:item`, `:stmt` and `:tt`, but it could be expanded in the future.

Fixes #24189.
Fixes #26444.
Fixes #27832.
Fixes #34030.
Fixes #35650.
Fixes #39964.
Fixes the 4th comment in #40569.
Fixes the issue blocking #40984.
@Mark-Simulacrum Mark-Simulacrum added the C-feature-request Category: A feature request, i.e: not implemented / a PR. label Jul 27, 2017
@iago-lito
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Is it also a good place to gather resources explaining why it is the case that macros don't backtrack?
I'm not particularly in need of bactracking, but I'm curious why this (no-)feature. I guess it's a limitation due to compiler optimization? Or maybe there are deeper theoretical problems with backtracking?

@jturner314 jturner314 mentioned this issue Sep 25, 2019
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@kmaork
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kmaork commented Nov 16, 2020

Is there any progress here?
Until this is solved, is there a recommended workaround for extracting the last element of a repetition?
The best I've got is reversing the whole repetition using a recursive macro and then extracting the first element.

@ehuss ehuss mentioned this issue Apr 16, 2021
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A-macros Area: All kinds of macros (custom derive, macro_rules!, proc macros, ..) C-feature-request Category: A feature request, i.e: not implemented / a PR.
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@Mark-Simulacrum @kmaork @iago-lito