Accept trivial consts based on trivial consts

[saethlin]

The previous implementation only accepted trivial consts that assign a literal. For example:

const A: usize = 0;
const B: usize = A;

Before this PR, only A was a trivial const. Now B is too.

[saethlin]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 2f43088 (2f430888c54e6073a1742439ac5020b07745922b, parent: 9ea8d67cc60e88ad6fffbf299a454c44227e001c)

[rust-timer]

Finished benchmarking commit (2f43088): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

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.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@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)	0.4%	[0.1%, 0.9%]	3
Improvements ✅ (primary)	-0.7%	[-1.1%, -0.2%]	13
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.7%	[-1.1%, -0.2%]	13

Max RSS (memory usage)

Results (primary 0.5%)

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

	mean	range	count
Regressions ❌ (primary)	2.0%	[0.6%, 3.3%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.4%	[-2.4%, -2.4%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.5%	[-2.4%, 3.3%]	3

Cycles

Results (primary -4.4%, secondary -2.1%)

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)	1.6%	[1.6%, 1.6%]	1
Improvements ✅ (primary)	-4.4%	[-5.1%, -3.6%]	2
Improvements ✅ (secondary)	-5.8%	[-5.8%, -5.8%]	1
All ❌✅ (primary)	-4.4%	[-5.1%, -3.6%]	2

Binary size

Results (primary -0.3%, secondary -0.1%)

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%]	2
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	2
Improvements ✅ (primary)	-0.4%	[-0.6%, -0.0%]	16
Improvements ✅ (secondary)	-0.1%	[-0.2%, -0.0%]	2
All ❌✅ (primary)	-0.3%	[-0.6%, 0.0%]	18

Bootstrap: 474.628s -> 474.127s (-0.11%)
Artifact size: 390.61 MiB -> 390.58 MiB (-0.01%)