Replace weak only lang items with a custom attribute

[Urgau]

When I added the invalid_runtime_symbol_definitions and suspicious_runtime_symbol_definitions lints in #155521 I added the concept of "weak only" lang item.

It's a LangItem that has no implementation, this is useful because the core symbols where not directly called, they were inserted by the compiler.

This mechanism work well, but T-lang approved the extension to "all extern functions referenced by the standard library", which makes the lang item approach impractical as we needs to update 5 files just to declare them. I'm not sure we can ask library contributor to do the dance for each extern "C" functions.

But the most problematic thing is that many extern functions in std come from libc not std, and adding lang items there is just no feasible I think.

Instead this PR proposed that we introduce a proper mechanism for them by adding a attribute #[rustc_canonical_symbol = "..."], which is encoded and decoded in rmeta like lang items and diagnostics items.

This simplifies the declaration as we only need to put the attribute to be effective #[rustc_canonical_symbol = "open"]. It also opens up many possibilities (none implemented here) like putting them on use statements (so we don't need to modify libc) or having the attribute be placed on a module. The table may also be useful on it's own if we want someday to do it for all crates, not just the standard library.

Follow up to #155521 and #158522

[rustbot]

Some changes occurred in compiler/rustc_hir/src/attrs

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred in compiler/rustc_passes/src/check_attr.rs

cc @jdonszelmann, @JonathanBrouwer

[rustbot]

r? @jackh726

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[mejrs]

What is the reason for the name field, can't this be a word attribute like #[rustc_canonical_symbol]? Only the tests have a case where the name is different from the item.

[Urgau]

What is the reason for the name field

Calling the symbol_name query can be quite expensive, in particular when done repeatedly, but you're right that the name field could probably just be deduced (and maybe should).

Let's do a perf run without it.
@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 9809522 (9809522494587bd84b1cae60f99fb7080a2a2ec1)
Base parent: 656ccbe (656ccbe796ff98def9b555c118e1620c5389e3b2)

[rust-timer]

Finished benchmarking commit (9809522): comparison URL.

Overall result: ❌ regressions - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@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)	0.3%	[0.1%, 0.6%]	10
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.4%	[-0.4%, -0.4%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -4.3%, secondary 0.0%)

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)	4.3%	[1.1%, 6.5%]	7
Improvements ✅ (primary)	-4.3%	[-6.2%, -2.5%]	3
Improvements ✅ (secondary)	-4.2%	[-6.0%, -2.0%]	7
All ❌✅ (primary)	-4.3%	[-6.2%, -2.5%]	3

Cycles

This perf run didn't have relevant results for this metric.

Binary size

Results (primary 0.0%, secondary 0.0%)

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

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.0%]	27
Regressions ❌ (secondary)	0.0%	[0.0%, 0.1%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.0%]	27

Bootstrap: 490.561s -> 491.765s (0.25%)
Artifact size: 390.05 MiB -> 389.50 MiB (-0.14%)

[Urgau]

I've switch to deducing the symbol name in the latest commit. Let's see if it has a perf impact.

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 4c682e3 (4c682e32f0cd928dd72d1e25402074a8cf953a8f)
Base parent: b26c8ef (b26c8ef0535b2de24a0af4048370caf449eebabd)

[rust-timer]

Finished benchmarking commit (4c682e3): comparison URL.

Overall result: ❌ regressions - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@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)	0.2%	[0.1%, 0.6%]	12
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.3%	[-0.3%, -0.3%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (secondary 1.2%)

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)	5.6%	[2.4%, 9.5%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.3%	[-5.3%, -2.1%]	4
All ❌✅ (primary)	-	-	0

Cycles

This perf run didn't have relevant results for this metric.

Binary size

Results (primary -0.0%, secondary -0.0%)

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)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-0.0%	[-0.0%, -0.0%]	10
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.0%]	4
All ❌✅ (primary)	-0.0%	[-0.0%, -0.0%]	10

Bootstrap: 492.122s -> 488.922s (-0.65%)
Artifact size: 389.50 MiB -> 389.47 MiB (-0.01%)