Rollup merge of rust-lang#141356 - dianne:thir-lower-params-before-body-expr, r=compiler-errors

lower bodies' params to thir before the body's value

Two motivations for this:
- Lowering params first means errors from lowering the params are emitted before errors from lowering the body's expression. This comes up in [tests/ui/associated-consts/associated-const-type-parameter-pattern.stderr](https://github.com/rust-lang/rust/compare/master...dianne:rust:thir-lower-params-before-body-expr?expand=1#diff-acac6ea10e991af0da91633e08b2739f9f9ca0c8f826401b6ba829914d0806f2), where both the params and expression encounter errors in translating consts to patterns. This change puts the errors in the order they appear in the source file.
- Guard patterns (rust-lang#129967) contain expressions, so lowering params containing guard patterns may add more expressions to the THIR. However, there's a check for `-Zunpretty=thir-tree` that the final expression in the THIR corresponds to its value [(link)](https://github.com/rust-lang/rust/blob/c43786c9b7b8d8dcc3f9c604e0e3074c16ed69d3/compiler/rustc_mir_build/src/builder/mod.rs#L453-L455); lowering params last would break this. As an alternative way to get guard patterns to work, I think the pretty-printer could use the expression returned by `thir_body` and the check could be removed or changed (rust-lang#141357).

Author: matthiaskrgr

compiler/rustc_mir_build/src/thir/cx/mod.rs

@@ -27,8 +27,8 @@ pub(crate) fn thir_body(
     if let Some(reported) = cx.typeck_results.tainted_by_errors {
         return Err(reported);
     }
-    let expr = cx.mirror_expr(body.value);
 
+    // Lower the params before the body's expression so errors from params are shown first.
     let owner_id = tcx.local_def_id_to_hir_id(owner_def);
     if let Some(fn_decl) = tcx.hir_fn_decl_by_hir_id(owner_id) {
         let closure_env_param = cx.closure_env_param(owner_def, owner_id);
@@ -48,6 +48,7 @@ pub(crate) fn thir_body(
         }
     }
 
+    let expr = cx.mirror_expr(body.value);
     Ok((tcx.alloc_steal_thir(cx.thir), expr))
 }
 

tests/ui/associated-consts/associated-const-type-parameter-pattern.stderr

@@ -27,31 +27,31 @@ LL |         B::X => println!("B::X"),
    |         ^^^^ `const` depends on a generic parameter
 
 error[E0158]: constant pattern cannot depend on generic parameters
-  --> $DIR/associated-const-type-parameter-pattern.rs:30:9
+  --> $DIR/associated-const-type-parameter-pattern.rs:28:48
    |
 LL | pub trait Foo {
    | -------------
 LL |     const X: EFoo;
    |     ------------- constant defined here
 ...
 LL | pub fn test_let_pat<A: Foo, B: Foo>(arg: EFoo, A::X: EFoo) {
-   |                     - constant depends on this generic parameter
-LL |
-LL |     let A::X = arg;
-   |         ^^^^ `const` depends on a generic parameter
+   |                     -                          ^^^^ `const` depends on a generic parameter
+   |                     |
+   |                     constant depends on this generic parameter
 
 error[E0158]: constant pattern cannot depend on generic parameters
-  --> $DIR/associated-const-type-parameter-pattern.rs:28:48
+  --> $DIR/associated-const-type-parameter-pattern.rs:30:9
    |
 LL | pub trait Foo {
    | -------------
 LL |     const X: EFoo;
    |     ------------- constant defined here
 ...
 LL | pub fn test_let_pat<A: Foo, B: Foo>(arg: EFoo, A::X: EFoo) {
-   |                     -                          ^^^^ `const` depends on a generic parameter
-   |                     |
-   |                     constant depends on this generic parameter
+   |                     - constant depends on this generic parameter
+LL |
+LL |     let A::X = arg;
+   |         ^^^^ `const` depends on a generic parameter
 
 error: aborting due to 4 previous errors