Rollup merge of rust-lang#142594 - mejrs:new_desugaring, r=chenyukang

Add DesugaringKind::FormatLiteral

Implements `DesugaringKind::FormatLiteral` to mark the FormatArgs desugaring of format literals. The main use for this is to stop yapping about about formatting parameters if we're not anywhere near a format literal. The other use case is to fix suggestions such as rust-lang#141350. It might also be useful for new or existing diagnostics that check whether they're in a format-like macro.

cc `@xizheyin` `@fmease`

Author: GuillaumeGomez

core/src/fmt/mod.rs

@@ -855,11 +855,13 @@ impl Display for Arguments<'_> {
 #[rustc_on_unimplemented(
     on(
         crate_local,
-        label = "`{Self}` cannot be formatted using `{{:?}}`",
         note = "add `#[derive(Debug)]` to `{Self}` or manually `impl {This} for {Self}`"
     ),
-    message = "`{Self}` doesn't implement `{This}`",
-    label = "`{Self}` cannot be formatted using `{{:?}}` because it doesn't implement `{This}`"
+    on(
+        from_desugaring = "FormatLiteral",
+        label = "`{Self}` cannot be formatted using `{{:?}}` because it doesn't implement `{This}`"
+    ),
+    message = "`{Self}` doesn't implement `{This}`"
 )]
 #[doc(alias = "{:?}")]
 #[rustc_diagnostic_item = "Debug"]
@@ -986,11 +988,14 @@ pub use macros::Debug;
         any(Self = "std::path::Path", Self = "std::path::PathBuf"),
         label = "`{Self}` cannot be formatted with the default formatter; call `.display()` on it",
         note = "call `.display()` or `.to_string_lossy()` to safely print paths, \
-                as they may contain non-Unicode data"
+                as they may contain non-Unicode data",
+    ),
+    on(
+        from_desugaring = "FormatLiteral",
+        note = "in format strings you may be able to use `{{:?}}` (or {{:#?}} for pretty-print) instead",
+        label = "`{Self}` cannot be formatted with the default formatter",
     ),
-    message = "`{Self}` doesn't implement `{This}`",
-    label = "`{Self}` cannot be formatted with the default formatter",
-    note = "in format strings you may be able to use `{{:?}}` (or {{:#?}} for pretty-print) instead"
+    message = "`{Self}` doesn't implement `{This}`"
 )]
 #[doc(alias = "{}")]
 #[rustc_diagnostic_item = "Display"]

std/src/macros.rs

@@ -363,7 +363,14 @@ macro_rules! dbg {
         match $val {
             tmp => {
                 $crate::eprintln!("[{}:{}:{}] {} = {:#?}",
-                    $crate::file!(), $crate::line!(), $crate::column!(), $crate::stringify!($val), &tmp);
+                    $crate::file!(),
+                    $crate::line!(),
+                    $crate::column!(),
+                    $crate::stringify!($val),
+                    // The `&T: Debug` check happens here (not in the format literal desugaring)
+                    // to avoid format literal related messages and suggestions.
+                    &&tmp as &dyn $crate::fmt::Debug,
+                );
                 tmp
             }
         }