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More detail when expecting expression but encountering bad macro argument On nested macro invocations where the same macro fragment changes fragment type from one to the next, point at the chain of invocations and at the macro fragment definition place, explaining that the change has occurred. Fix #71039. ``` error: expected expression, found pattern `1 + 1` --> $DIR/trace_faulty_macros.rs:49:37 | LL | (let $p:pat = $e:expr) => {test!(($p,$e))}; | ------- -- this is interpreted as expression, but it is expected to be pattern | | | this macro fragment matcher is expression ... LL | (($p:pat, $e:pat)) => {let $p = $e;}; | ------ ^^ expected expression | | | this macro fragment matcher is pattern ... LL | test!(let x = 1+1); | ------------------ | | | | | this is expected to be expression | in this macro invocation | = note: when forwarding a matched fragment to another macro-by-example, matchers in the second macro will see an opaque AST of the fragment type, not the underlying tokens = note: this error originates in the macro `test` (in Nightly builds, run with -Z macro-backtrace for more info) ``` |
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| README.md | ||
The rustc_ast crate contains those things concerned purely with syntax
– that is, the AST ("abstract syntax tree"), along with some definitions for tokens and token streams, data structures/traits for mutating ASTs, and shared definitions for other AST-related parts of the compiler (like the lexer and macro-expansion).
For more information about how these things work in rustc, see the rustc dev guide: