checks/errors: Fix non-exhaustive pattern matching for literals

The compiler previously treated literal patterns (like 0, 'a', or true) as
wildcards, causing the exhaustiveness checker to incorrectly accept incomplete
match blocks.

This patch modifies the pattern analysis to lower literals into integer ranges.
It introduces a new helper  which uses
(for -fno-exceptions compatibility) to parse integers. It also updates
 to correctly handle finite boolean universes and default
other scalars (char, int) to wildcards without incorrectly inserting struct
constructors.

Additionally,  and
are updated to handle  constructors, preventing Internal Compiler
Errors (ICEs). Unicode and hex escape patterns in characters currently emit
a sorry diagnostic as they are not yet fully supported.

Fixes #4296

gcc/rust/ChangeLog:

	* checks/errors/rust-hir-pattern-analysis.h (Constructor::make_int_range):
	Add helper to create integer range constructors.
	* checks/errors/rust-hir-pattern-analysis.cc (lower_literal_pattern):
	New helper function to lower literals to integer ranges.
	(lower_pattern): Use new lower_literal_pattern helper.
	(parse_check_literal): Refactor for readability and base detection.
	(split_constructors): Handle integer, bool, and char types correctly
	and remove redundant struct constructors for primitives.
	(WitnessPat::to_string): Handle INT_RANGE to prevent ICE.
	(WitnessMatrix::apply_constructor): Handle INT_RANGE to prevent ICE.

gcc/testsuite/ChangeLog:

	* rust/compile/issue-4296.rs: New test.
	* rust/compile/issue-3929-2.rs: Add wildcard arm to match block.
	* rust/execute/torture/struct-pattern-match.rs: Add wildcard arm.

Signed-off-by: Jayant Chauhan <[email protected]>

Author: Pasta-coder

gcc/rust/checks/errors/rust-hir-pattern-analysis.cc

@@ -1245,7 +1245,7 @@ lower_tuple_pattern (Resolver::TypeCheckContext *ctx,
 	      variant->get_fields ().at (i++)->get_field_type ()));
 	  }
 	while (i < variant->num_fields ()
-		   - items_has_rest.get_upper_patterns ().size ())
+		     - items_has_rest.get_upper_patterns ().size ())
 	  {
 	    fields.push_back (
 	      DeconstructedPat::make_wildcard (pattern.get_locus ()));

gcc/testsuite/rust/compile/issue-4296.rs

@@ -1,3 +1,5 @@
+#![feature(no_core)]
+#![no_core]
 // { dg-options "-w" }
 pub fn main() {
     let x = 3;