Rollup merge of #70970 - eddyb:trait-vs-impl-mismatch, r=oli-obk

Detect mistyped associated consts in `Instance::resolve`.

*Based on #71049 to prevent redundant/misleading downstream errors.*

Fixes #70942 by refusing to resolve an associated `const` if it doesn't have the same type in the `impl` that it does in the `trait` (which we assume had errored, and `delay_span_bug` guards against bugs).

src/test/ui/consts/issue-70942-trait-vs-impl-mismatch.rs

@@ -0,0 +1,14 @@
+trait Nat {
+    const VALUE: usize;
+}
+
+struct Zero;
+
+impl Nat for Zero {
+    const VALUE: i32 = 0;
+    //~^ ERROR implemented const `VALUE` has an incompatible type for trait
+}
+
+fn main() {
+    let _: [i32; Zero::VALUE] = [];
+}

src/test/ui/consts/issue-70942-trait-vs-impl-mismatch.stderr

@@ -0,0 +1,12 @@
+error[E0326]: implemented const `VALUE` has an incompatible type for trait
+  --> $DIR/issue-70942-trait-vs-impl-mismatch.rs:8:18
+   |
+LL |     const VALUE: usize;
+   |                  ----- type in trait
+...
+LL |     const VALUE: i32 = 0;
+   |                  ^^^ expected `usize`, found `i32`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0326`.

src/test/ui/issues/issue-69602-type-err-during-codegen-ice.rs

@@ -19,5 +19,4 @@ impl TraitB for B { //~ ERROR not all trait items implemented, missing: `MyA`
 
 fn main() {
     let _ = [0; B::VALUE];
-    //~^ ERROR constant expression depends on a generic parameter
 }

src/test/ui/issues/issue-69602-type-err-during-codegen-ice.stderr

@@ -13,15 +13,7 @@ LL |     type MyA: TraitA;
 LL | impl TraitB for B {
    | ^^^^^^^^^^^^^^^^^ missing `MyA` in implementation
 
-error: constant expression depends on a generic parameter
-  --> $DIR/issue-69602-type-err-during-codegen-ice.rs:21:17
-   |
-LL |     let _ = [0; B::VALUE];
-   |                 ^^^^^^^^
-   |
-   = note: this may fail depending on what value the parameter takes
-
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
 Some errors have detailed explanations: E0046, E0437.
 For more information about an error, try `rustc --explain E0046`.