suggest fix for attempted integer identifier in patterns

compiler/rustc_error_messages/locales/en-US/mir_build.ftl

@@ -364,3 +364,5 @@ mir_build_suggest_let_else = you might want to use `let else` to handle the {$co
         [one] variant that isn't
         *[other] variants that aren't
     } matched
+
+mir_build_suggest_attempted_int_lit = alternatively, you could prepend the pattern with an underscore to define a new named variable; identifiers cannot begin with digits

compiler/rustc_mir_build/src/errors.rs

@@ -770,6 +770,8 @@ pub(crate) struct PatternNotCovered<'s, 'tcx> {
     #[subdiagnostic]
     pub let_suggestion: Option<SuggestLet>,
     #[subdiagnostic]
+    pub misc_suggestion: Option<MiscPatternSuggestion>,
+    #[subdiagnostic]
     pub res_defined_here: Option<ResDefinedHere>,
 }
 
@@ -848,3 +850,16 @@ pub enum SuggestLet {
         count: usize,
     },
 }
+
+#[derive(Subdiagnostic)]
+pub enum MiscPatternSuggestion {
+    #[suggestion(
+        mir_build_suggest_attempted_int_lit,
+        code = "_",
+        applicability = "maybe-incorrect"
+    )]
+    AttemptedIntegerLiteral {
+        #[primary_span]
+        start_span: Span,
+    },
+}

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -6,8 +6,9 @@ use super::{PatCtxt, PatternError};
 
 use crate::errors::*;
 
+use hir::{ExprKind, PatKind};
 use rustc_arena::TypedArena;
-use rustc_ast::Mutability;
+use rustc_ast::{LitKind, Mutability};
 use rustc_errors::{
     struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed, MultiSpan,
 };
@@ -389,7 +390,7 @@ impl<'p, 'tcx> MatchVisitor<'_, 'p, 'tcx> {
             return;
         }
 
-        let (inform, interpreted_as_const, res_defined_here,let_suggestion) =
+        let (inform, interpreted_as_const, res_defined_here,let_suggestion, misc_suggestion) =
             if let hir::PatKind::Path(hir::QPath::Resolved(
                 None,
                 hir::Path {
@@ -413,6 +414,7 @@ impl<'p, 'tcx> MatchVisitor<'_, 'p, 'tcx> {
                         }
                     },
                     None,
+                    None,
                 )
             } else if let Some(span) = sp && self.tcx.sess.source_map().is_span_accessible(span) {
                 let mut bindings = vec![];
@@ -426,10 +428,19 @@ impl<'p, 'tcx> MatchVisitor<'_, 'p, 'tcx> {
                 let end_span = semi_span.shrink_to_lo();
                 let count = witnesses.len();
 
+                // If the pattern to match is an integer literal:
+                let int_suggestion = if
+                    let PatKind::Lit(expr) = &pat.kind
+                    && bindings.is_empty()
+                    && let ExprKind::Lit(Spanned { node: LitKind::Int(_, _), span }) = expr.kind {
+                    // Then give a suggestion, the user might've meant to create a binding instead.
+                    Some(MiscPatternSuggestion::AttemptedIntegerLiteral { start_span: span.shrink_to_lo() })
+                } else { None };
+
                 let let_suggestion = if bindings.is_empty() {SuggestLet::If{start_span, semi_span, count}} else{ SuggestLet::Else{end_span, count }};
-                (sp.map(|_|Inform), None, None, Some(let_suggestion))
+                (sp.map(|_|Inform), None, None, Some(let_suggestion), int_suggestion)
             } else{
-                (sp.map(|_|Inform), None, None,  None)
+                (sp.map(|_|Inform), None, None,  None, None)
             };
 
         let adt_defined_here = try {
@@ -453,6 +464,7 @@ impl<'p, 'tcx> MatchVisitor<'_, 'p, 'tcx> {
             _p: (),
             pattern_ty,
             let_suggestion,
+            misc_suggestion,
             res_defined_here,
             adt_defined_here,
         });

tests/ui/consts/const-match-check.eval1.stderr

@@ -11,6 +11,10 @@ help: you might want to use `if let` to ignore the variants that aren't matched
    |
 LL |     A = { if let 0 = 0 { todo!() } 0 },
    |           ++           ~~~~~~~~~~~
+help: alternatively, you could prepend the pattern with an underscore to define a new named variable; identifiers cannot begin with digits
+   |
+LL |     A = { let _0 = 0; 0 },
+   |               +
 
 error: aborting due to previous error
 

tests/ui/consts/const-match-check.eval2.stderr

@@ -11,6 +11,10 @@ help: you might want to use `if let` to ignore the variants that aren't matched
    |
 LL |     let x: [i32; { if let 0 = 0 { todo!() } 0 }] = [];
    |                    ++           ~~~~~~~~~~~
+help: alternatively, you could prepend the pattern with an underscore to define a new named variable; identifiers cannot begin with digits
+   |
+LL |     let x: [i32; { let _0 = 0; 0 }] = [];
+   |                        +
 
 error: aborting due to previous error
 

tests/ui/consts/const-match-check.matchck.stderr

@@ -11,6 +11,10 @@ help: you might want to use `if let` to ignore the variants that aren't matched
    |
 LL | const X: i32 = { if let 0 = 0 { todo!() } 0 };
    |                  ++           ~~~~~~~~~~~
+help: alternatively, you could prepend the pattern with an underscore to define a new named variable; identifiers cannot begin with digits
+   |
+LL | const X: i32 = { let _0 = 0; 0 };
+   |                      +
 
 error[E0005]: refutable pattern in local binding
   --> $DIR/const-match-check.rs:8:23
@@ -25,6 +29,10 @@ help: you might want to use `if let` to ignore the variants that aren't matched
    |
 LL | static Y: i32 = { if let 0 = 0 { todo!() } 0 };
    |                   ++           ~~~~~~~~~~~
+help: alternatively, you could prepend the pattern with an underscore to define a new named variable; identifiers cannot begin with digits
+   |
+LL | static Y: i32 = { let _0 = 0; 0 };
+   |                       +
 
 error[E0005]: refutable pattern in local binding
   --> $DIR/const-match-check.rs:13:26
@@ -39,6 +47,10 @@ help: you might want to use `if let` to ignore the variants that aren't matched
    |
 LL |     const X: i32 = { if let 0 = 0 { todo!() } 0 };
    |                      ++           ~~~~~~~~~~~
+help: alternatively, you could prepend the pattern with an underscore to define a new named variable; identifiers cannot begin with digits
+   |
+LL |     const X: i32 = { let _0 = 0; 0 };
+   |                          +
 
 error[E0005]: refutable pattern in local binding
   --> $DIR/const-match-check.rs:19:26
@@ -53,6 +65,10 @@ help: you might want to use `if let` to ignore the variants that aren't matched
    |
 LL |     const X: i32 = { if let 0 = 0 { todo!() } 0 };
    |                      ++           ~~~~~~~~~~~
+help: alternatively, you could prepend the pattern with an underscore to define a new named variable; identifiers cannot begin with digits
+   |
+LL |     const X: i32 = { let _0 = 0; 0 };
+   |                          +
 
 error: aborting due to 4 previous errors
 

tests/ui/pattern/issue-106552.rs

@@ -0,0 +1,7 @@
+fn main() {
+    let 5 = 6;
+    //~^ error refutable pattern in local binding [E0005]
+
+    let x @ 5 = 6;
+    //~^ error refutable pattern in local binding [E0005]
+}

tests/ui/pattern/issue-106552.stderr

@@ -0,0 +1,35 @@
+error[E0005]: refutable pattern in local binding
+  --> $DIR/issue-106552.rs:2:9
+   |
+LL |     let 5 = 6;
+   |         ^ patterns `i32::MIN..=4_i32` and `6_i32..=i32::MAX` not covered
+   |
+   = note: `let` bindings require an "irrefutable pattern", like a `struct` or an `enum` with only one variant
+   = note: for more information, visit https://doc.rust-lang.org/book/ch18-02-refutability.html
+   = note: the matched value is of type `i32`
+help: you might want to use `if let` to ignore the variants that aren't matched
+   |
+LL |     if let 5 = 6 { todo!() }
+   |     ++           ~~~~~~~~~~~
+help: alternatively, you could prepend the pattern with an underscore to define a new named variable; identifiers cannot begin with digits
+   |
+LL |     let _5 = 6;
+   |         +
+
+error[E0005]: refutable pattern in local binding
+  --> $DIR/issue-106552.rs:5:9
+   |
+LL |     let x @ 5 = 6;
+   |         ^^^^^ patterns `i32::MIN..=4_i32` and `6_i32..=i32::MAX` not covered
+   |
+   = note: `let` bindings require an "irrefutable pattern", like a `struct` or an `enum` with only one variant
+   = note: for more information, visit https://doc.rust-lang.org/book/ch18-02-refutability.html
+   = note: the matched value is of type `i32`
+help: you might want to use `let else` to handle the variants that aren't matched
+   |
+LL |     let x @ 5 = 6 else { todo!() };
+   |                   ++++++++++++++++
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0005`.