clean up layout error diagnostics

- group the fluent slugs together
- reword (internal-only) "too generic" error to be more in line with
  the other errors

compiler/rustc_middle/messages.ftl

@@ -37,9 +37,6 @@ middle_autodiff_unsafe_inner_const_ref = reading from a `Duplicated` const {$ty}
 middle_bounds_check =
     index out of bounds: the length is {$len} but the index is {$index}
 
-middle_cannot_be_normalized =
-    unable to determine layout for `{$ty}` because `{$failure_ty}` cannot be normalized
-
 middle_conflict_types =
     this expression supplies two conflicting concrete types for the same opaque type
 
@@ -52,9 +49,6 @@ middle_const_eval_non_int =
 middle_const_not_used_in_type_alias =
     const parameter `{$ct}` is part of concrete type but not used in parameter list for the `impl Trait` type alias
 
-middle_cycle =
-    a cycle occurred during layout computation
-
 middle_deprecated = use of deprecated {$kind} `{$path}`{$has_note ->
         [true] : {$note}
         *[other] {""}
@@ -78,9 +72,23 @@ middle_erroneous_constant = erroneous constant encountered
 middle_failed_writing_file =
     failed to write file {$path}: {$error}"
 
+middle_layout_cycle =
+    a cycle occurred during layout computation
+
+middle_layout_normalization_failure =
+    unable to determine layout for `{$ty}` because `{$failure_ty}` cannot be normalized
+
 middle_layout_references_error =
     the type has an unknown layout
 
+middle_layout_size_overflow =
+    values of the type `{$ty}` are too big for the target architecture
+
+middle_layout_too_generic = the type `{$ty}` does not have a fixed layout
+
+middle_layout_unknown =
+    the type `{$ty}` has an unknown layout
+
 middle_opaque_hidden_type_mismatch =
     concrete type differs from previous defining opaque type use
     .label = expected `{$self_ty}`, got `{$other_ty}`
@@ -98,16 +106,8 @@ middle_strict_coherence_needs_negative_coherence =
     to use `strict_coherence` on this trait, the `with_negative_coherence` feature must be enabled
     .label = due to this attribute
 
-middle_too_generic = `{$ty}` does not have a fixed size
-
 middle_type_length_limit = reached the type-length limit while instantiating `{$shrunk}`
 
-middle_unknown_layout =
-    the type `{$ty}` has an unknown layout
-
 middle_unsupported_union = we don't support unions yet: '{$ty_name}'
 
-middle_values_too_big =
-    values of the type `{$ty}` are too big for the target architecture
-
 middle_written_to_path = the full type name has been written to '{$path}'

compiler/rustc_middle/src/error.rs

@@ -132,19 +132,19 @@ impl fmt::Debug for CustomSubdiagnostic<'_> {
 
 #[derive(Diagnostic)]
 pub enum LayoutError<'tcx> {
-    #[diag(middle_unknown_layout)]
+    #[diag(middle_layout_unknown)]
     Unknown { ty: Ty<'tcx> },
 
-    #[diag(middle_too_generic)]
+    #[diag(middle_layout_too_generic)]
     TooGeneric { ty: Ty<'tcx> },
 
-    #[diag(middle_values_too_big)]
+    #[diag(middle_layout_size_overflow)]
     Overflow { ty: Ty<'tcx> },
 
-    #[diag(middle_cannot_be_normalized)]
+    #[diag(middle_layout_normalization_failure)]
     NormalizationFailure { ty: Ty<'tcx>, failure_ty: String },
 
-    #[diag(middle_cycle)]
+    #[diag(middle_layout_cycle)]
     Cycle,
 
     #[diag(middle_layout_references_error)]

compiler/rustc_middle/src/ty/layout.rs

@@ -264,11 +264,11 @@ impl<'tcx> LayoutError<'tcx> {
 
         use crate::fluent_generated::*;
         match self {
-            Unknown(_) => middle_unknown_layout,
-            SizeOverflow(_) => middle_values_too_big,
-            TooGeneric(_) => middle_too_generic,
-            NormalizationFailure(_, _) => middle_cannot_be_normalized,
-            Cycle(_) => middle_cycle,
+            Unknown(_) => middle_layout_unknown,
+            SizeOverflow(_) => middle_layout_size_overflow,
+            TooGeneric(_) => middle_layout_too_generic,
+            NormalizationFailure(_, _) => middle_layout_normalization_failure,
+            Cycle(_) => middle_layout_cycle,
             ReferencesError(_) => middle_layout_references_error,
         }
     }
@@ -297,7 +297,7 @@ impl<'tcx> fmt::Display for LayoutError<'tcx> {
         match *self {
             LayoutError::Unknown(ty) => write!(f, "the type `{ty}` has an unknown layout"),
             LayoutError::TooGeneric(ty) => {
-                write!(f, "`{ty}` does not have a fixed size")
+                write!(f, "the type `{ty}` does not have a fixed layout")
             }
             LayoutError::SizeOverflow(ty) => {
                 write!(f, "values of the type `{ty}` are too big for the target architecture")

tests/ui/layout/debug.rs

@@ -86,4 +86,4 @@ union EmptyUnion {} //~ ERROR: has an unknown layout
 // Test the error message of `LayoutError::TooGeneric`
 // (this error is never emitted to users).
 #[rustc_layout(debug)]
-type TooGeneric<T> = T; //~ ERROR: does not have a fixed size
+type TooGeneric<T> = T; //~ ERROR: does not have a fixed layout

tests/ui/layout/debug.stderr

@@ -596,7 +596,7 @@ error: the type has an unknown layout
 LL | union EmptyUnion {}
    | ^^^^^^^^^^^^^^^^
 
-error: `T` does not have a fixed size
+error: the type `T` does not have a fixed layout
   --> $DIR/debug.rs:89:1
    |
 LL | type TooGeneric<T> = T;