make size_of_val and align_of_val const fns

[Gankra]

Closes #46571

[rust-highfive]

r? @Mark-Simulacrum

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[rust-highfive]

The job x86_64-gnu-llvm-3.9 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

[00:04:50] travis_fold:start:tidy
travis_time:start:tidy
tidy check
[00:04:51] tidy error: /checkout/src/librustc_mir/transform/qualify_consts.rs:872: trailing whitespace
[00:04:51] tidy error: /checkout/src/librustc_mir/transform/qualify_consts.rs:873: trailing whitespace
[00:04:51] tidy error: /checkout/src/librustc_mir/transform/qualify_consts.rs:874: trailing whitespace
[00:04:52] some tidy checks failed
[00:04:52] 
[00:04:52] 
[00:04:52] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/checkout/src" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "--no-vendor" "--quiet"
[00:04:52] 
[00:04:52] 
[00:04:52] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:04:52] Build completed unsuccessfully in 0:01:59
[00:04:52] Build completed unsuccessfully in 0:01:59
[00:04:52] make: *** [tidy] Error 1
[00:04:52] Makefile:79: recipe for target 'tidy' failed

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:1e0be218
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[oli-obk]

If you want this PR to go through fast, you should be marking the function with #[rustc_const_unstable(feature = "const_size_of_val")] (and create a tracking issue)

[kennytm]

@oli-obk const fn features have no tracking issues for now, unfortunately. See #44644.

[oli-obk]

you should be able to write

static FOO: &[u8] = &OWO;
static BAR: usize = size_of_val(FOO);

because coercions don't need method calls like slice indexing does

[kennytm]

I've 👎'ed this PR due to #46571 (comment)

[oli-obk]

tagging @rust-lang/lang because making this const fn would complicate future language changes.

[Gankra]

Closing in favour of #50579 which I will use to make slice.len() const.