[gcc/devel/rust/master] Merge #1315

[gcc/devel/rust/master] Merge #1315

[Thomas Schwinge]

https://gcc.gnu.org/g:848a1a28b91d105ff4f21cc9993befbcecb3e39d

commit 848a1a28b91d105ff4f21cc9993befbcecb3e39d
Merge: d4a0780073b 0dbfdb5cfc5
Author: bors[bot] <26634292+bors[bot]@users.noreply.github.com>
Date:   Fri Jun 17 07:12:29 2022 +0000

    Merge #1315
    
    1315: Add base for parsing const generic application r=CohenArthur a=CohenArthur
    
    1. Refactor const generic declaration
    
    The default value for const generics now benefits from using the same
    function as parsing a regular const generic expression
    
    2. `Parser::parse_type` should not always add errors
    
    In the case that we are parsing a const generic and not a type, we
    should not emit bogus errors from `parse_type` such as "unexpected token
    in type: LITERAL". Thus, we add a flag to the function to not always
    add errors to the error table
    
    3. Figure out how to deal with ambiguities
    
    In the following cases, parsing is ambiguous:
    
    ```rust
    let a: Foo<N>;
    ```
    
    What is N? Is it a type to be used as a generic argument? Is it a const
    value to be used for a const generic argument? We need to keep both
    possibilities and differentiate later during typechecking. We need to
    figure out if it would be better to keep the ambiguity in our future
    `ConstGenericArg` type (something like Kind::ConstVarOrType) or modify
    our current `AST::Type` to maybe get differentiated later as a const
    variable, which seems more annoying.
    
    Finally, since the const evaluation is not implemented yet, we are
    getting some bogus errors in the testcase. This commit simply serves as
    a necessary base: parsing const generics before we can apply them.
    
    
    Co-authored-by: Arthur Cohen <arthur.cohen@embecosm.com>

Diff:

 gcc/rust/ast/rust-path.h                       |   2 +
 gcc/rust/parse/rust-parse-impl.h               | 159 +++++++++++++++----------
 gcc/rust/parse/rust-parse.h                    |   3 +-
 gcc/testsuite/rust/compile/const_generics_3.rs |  26 ++++
 4 files changed, 128 insertions(+), 62 deletions(-)