Auto merge of #103217 - mejrs:track, r=eholk

Track where diagnostics were created.

This implements the `-Ztrack-diagnostics` flag, which uses `#[track_caller]` to track where diagnostics are created. It is meant as a debugging tool much like `-Ztreat-err-as-bug`.

For example, the following code...

```rust
struct A;
struct B;

fn main(){
    let _: A = B;
}
```
...now emits the following error message:

```
error[E0308]: mismatched types
 --> src\main.rs:5:16
  |
5 |     let _: A = B;
  |            -   ^ expected struct `A`, found struct `B`
  |            |
  |            expected due to this
-Ztrack-diagnostics: created at compiler\rustc_infer\src\infer\error_reporting\mod.rs:2275:31
```

compiler/rustc_errors/src/diagnostic.rs

@@ -12,6 +12,7 @@ use rustc_span::{Span, DUMMY_SP};
 use std::borrow::Cow;
 use std::fmt;
 use std::hash::{Hash, Hasher};
+use std::panic::Location;
 
 /// Error type for `Diagnostic`'s `suggestions` field, indicating that
 /// `.disable_suggestions()` was called on the `Diagnostic`.
@@ -107,6 +108,31 @@ pub struct Diagnostic {
     /// If diagnostic is from Lint, custom hash function ignores notes
     /// otherwise hash is based on the all the fields
     pub is_lint: bool,
+
+    /// With `-Ztrack_diagnostics` enabled,
+    /// we print where in rustc this error was emitted.
+    pub emitted_at: DiagnosticLocation,
+}
+
+#[derive(Clone, Debug, Encodable, Decodable)]
+pub struct DiagnosticLocation {
+    file: Cow<'static, str>,
+    line: u32,
+    col: u32,
+}
+
+impl DiagnosticLocation {
+    #[track_caller]
+    fn caller() -> Self {
+        let loc = Location::caller();
+        DiagnosticLocation { file: loc.file().into(), line: loc.line(), col: loc.column() }
+    }
+}
+
+impl fmt::Display for DiagnosticLocation {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}:{}:{}", self.file, self.line, self.col)
+    }
 }
 
 #[derive(Clone, Debug, PartialEq, Eq, Hash, Encodable, Decodable)]
@@ -173,10 +199,12 @@ impl StringPart {
 }
 
 impl Diagnostic {
+    #[track_caller]
     pub fn new<M: Into<DiagnosticMessage>>(level: Level, message: M) -> Self {
         Diagnostic::new_with_code(level, None, message)
     }
 
+    #[track_caller]
     pub fn new_with_code<M: Into<DiagnosticMessage>>(
         level: Level,
         code: Option<DiagnosticId>,
@@ -192,6 +220,7 @@ impl Diagnostic {
             args: Default::default(),
             sort_span: DUMMY_SP,
             is_lint: false,
+            emitted_at: DiagnosticLocation::caller(),
         }
     }