CTFE: avoid emitting a hard error on generic normalization failures

[lcnr]

Fixes #149081.

The fix is quite unsatisfying and should not be necessary, cc #149283. That change is significantly more involved. This temporary fix introduces some unnecessary complexity and may hide other type system bugs.

cc @rust-lang/types I think we should try to fix issue #149283 in the near future and then remove this hack again.

I originally intended a more targeted fix. I wanted to skip evaluating constants in MIR opts if their body was polymorphic and the current generic arguments still reference generic parameters. Notes from looking into this:

• we only fetch the MIR in the eval_to_allocation_raw query
• figuring out which MIR to use is hard
  • dealing with trivial consts is annoying
  • need to resolve instances for associated consts
  • implementing this by hand is hard
• inlining handles this issue by bailing on literally all normalization failures, even the ones that imply an unsoundness
  • try_normalize_after_erasing_regions generally does two things
    • deal with ambiguity after inlining
    • deal with error tainting issues (please don't, we should stop doing that)
• CTFE could be changed to always silently ignore normalization failures if we're in a generic body
  • hides actual bugs <- went with this option

r? types

[rustbot]

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

warning ../../../../../package.json: License should be a valid SPDX license expression
warning No license field
[1/4] Resolving packages...
[2/4] Fetching packages...
error Error: https://registry.yarnpkg.com/ms/-/ms-2.1.3.tgz: Request failed "500 Internal Server Error"
    at ResponseError.ExtendableBuiltin (/node/lib/node_modules/yarn/lib/cli.js:696:66)
    at new ResponseError (/node/lib/node_modules/yarn/lib/cli.js:802:124)
    at Request.<anonymous> (/node/lib/node_modules/yarn/lib/cli.js:66750:16)
    at Request.emit (node:events:518:28)
    at module.exports.Request.onRequestResponse (/node/lib/node_modules/yarn/lib/cli.js:142287:10)
    at ClientRequest.emit (node:events:518:28)
    at HTTPParser.parserOnIncomingClient (node:_http_client:698:27)
    at HTTPParser.parserOnHeadersComplete (node:_http_common:119:17)
    at TLSSocket.socketOnData (node:_http_client:540:22)
    at TLSSocket.emit (node:events:518:28)
info Visit https://yarnpkg.com/en/docs/cli/install for documentation about this command.
yarn install did not exit successfully

thread 'main' (59568) panicked at src/tools/rustdoc-gui-test/src/main.rs:63:10:
unable to install browser-ui-test: Custom { kind: Other, error: "yarn install returned exit code exit status: 1" }
stack backtrace:
   0: __rustc::rust_begin_unwind
             at /rustc/3b4dd9bf1410f8da6329baa36ce5e37673cbbd1f/library/std/src/panicking.rs:698:5
   1: core::panicking::panic_fmt
             at /rustc/3b4dd9bf1410f8da6329baa36ce5e37673cbbd1f/library/core/src/panicking.rs:80:14

[RalfJung]

😢
But I can live with this as a temporary work-around.