1bfd987f69
Report an error if a lang item has the wrong number of generic arguments This pull request fixes #83893. The issue is that the lang item code currently checks whether the lang item has the correct item kind (e.g. a `#[lang="add"]` has to be a trait), but not whether the item has the correct number of generic arguments. This can lead to an "index out of bounds" ICE when the compiler tries to create more substitutions than there are suitable types available (if the lang item was declared with too many generic arguments). For instance, here is a reduced ("reduced" in the sense that it does not trigger additional errors) version of the example given in #83893: ```rust #![feature(lang_items,no_core)] #![no_core] #![crate_type="lib"] #[lang = "sized"] trait MySized {} #[lang = "add"] trait MyAdd<'a, T> {} fn ice() { let r = 5; let a = 6; r + a } ``` On current nightly, this immediately causes an ICE without any warnings or errors emitted. With the changes in this PR, however, I get no ICE and two errors: ``` error[E0718]: `add` language item must be applied to a trait with 1 generic argument --> pr-ex.rs:8:1 | 8 | #[lang = "add"] | ^^^^^^^^^^^^^^^ 9 | trait MyAdd<'a, T> {} | ------- this trait has 2 generic arguments, not 1 error[E0369]: cannot add `{integer}` to `{integer}` --> pr-ex.rs:14:7 | 14 | r + a | - ^ - {integer} | | | {integer} error: aborting due to 2 previous errors Some errors have detailed explanations: E0369, E0718. For more information about an error, try `rustc --explain E0369`. ``` |
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For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.