Use token::Lit in ast::ExprKind::Lit.

Instead of `ast::Lit`.

Literal lowering now happens at two different times. Expression literals
are lowered when HIR is crated. Attribute literals are lowered during
parsing.

This commit changes the language very slightly. Some programs that used
to not compile now will compile. This is because some invalid literals
that are removed by `cfg` or attribute macros will no longer trigger
errors. See this comment for more details:
https://github.com/rust-lang/rust/pull/102944#issuecomment-1277476773

compiler/rustc_session/src/errors.rs

@@ -1,6 +1,9 @@
 use std::num::NonZeroU32;
 
 use crate::cgu_reuse_tracker::CguReuse;
+use crate::parse::ParseSess;
+use rustc_ast::token;
+use rustc_ast::util::literal::LitError;
 use rustc_errors::MultiSpan;
 use rustc_macros::Diagnostic;
 use rustc_span::{Span, Symbol};
@@ -191,3 +194,162 @@ pub enum UnleashedFeatureHelp {
         span: Span,
     },
 }
+
+#[derive(Diagnostic)]
+#[diag(parser_invalid_literal_suffix)]
+pub(crate) struct InvalidLiteralSuffix {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    // FIXME(#100717)
+    pub kind: String,
+    pub suffix: Symbol,
+}
+
+#[derive(Diagnostic)]
+#[diag(parser_invalid_int_literal_width)]
+#[help]
+pub(crate) struct InvalidIntLiteralWidth {
+    #[primary_span]
+    pub span: Span,
+    pub width: String,
+}
+
+#[derive(Diagnostic)]
+#[diag(parser_invalid_num_literal_base_prefix)]
+#[note]
+pub(crate) struct InvalidNumLiteralBasePrefix {
+    #[primary_span]
+    #[suggestion(applicability = "maybe-incorrect", code = "{fixed}")]
+    pub span: Span,
+    pub fixed: String,
+}
+
+#[derive(Diagnostic)]
+#[diag(parser_invalid_num_literal_suffix)]
+#[help]
+pub(crate) struct InvalidNumLiteralSuffix {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    pub suffix: String,
+}
+
+#[derive(Diagnostic)]
+#[diag(parser_invalid_float_literal_width)]
+#[help]
+pub(crate) struct InvalidFloatLiteralWidth {
+    #[primary_span]
+    pub span: Span,
+    pub width: String,
+}
+
+#[derive(Diagnostic)]
+#[diag(parser_invalid_float_literal_suffix)]
+#[help]
+pub(crate) struct InvalidFloatLiteralSuffix {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    pub suffix: String,
+}
+
+#[derive(Diagnostic)]
+#[diag(parser_int_literal_too_large)]
+pub(crate) struct IntLiteralTooLarge {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parser_hexadecimal_float_literal_not_supported)]
+pub(crate) struct HexadecimalFloatLiteralNotSupported {
+    #[primary_span]
+    #[label(parser_not_supported)]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parser_octal_float_literal_not_supported)]
+pub(crate) struct OctalFloatLiteralNotSupported {
+    #[primary_span]
+    #[label(parser_not_supported)]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parser_binary_float_literal_not_supported)]
+pub(crate) struct BinaryFloatLiteralNotSupported {
+    #[primary_span]
+    #[label(parser_not_supported)]
+    pub span: Span,
+}
+
+pub fn report_lit_error(sess: &ParseSess, err: LitError, lit: token::Lit, span: Span) {
+    // Checks if `s` looks like i32 or u1234 etc.
+    fn looks_like_width_suffix(first_chars: &[char], s: &str) -> bool {
+        s.len() > 1 && s.starts_with(first_chars) && s[1..].chars().all(|c| c.is_ascii_digit())
+    }
+
+    // Try to lowercase the prefix if it's a valid base prefix.
+    fn fix_base_capitalisation(s: &str) -> Option<String> {
+        if let Some(stripped) = s.strip_prefix('B') {
+            Some(format!("0b{stripped}"))
+        } else if let Some(stripped) = s.strip_prefix('O') {
+            Some(format!("0o{stripped}"))
+        } else if let Some(stripped) = s.strip_prefix('X') {
+            Some(format!("0x{stripped}"))
+        } else {
+            None
+        }
+    }
+
+    let token::Lit { kind, suffix, .. } = lit;
+    match err {
+        // `LexerError` is an error, but it was already reported
+        // by lexer, so here we don't report it the second time.
+        LitError::LexerError => {}
+        LitError::InvalidSuffix => {
+            if let Some(suffix) = suffix {
+                sess.emit_err(InvalidLiteralSuffix {
+                    span,
+                    kind: format!("{}", kind.descr()),
+                    suffix,
+                });
+            }
+        }
+        LitError::InvalidIntSuffix => {
+            let suf = suffix.expect("suffix error with no suffix");
+            let suf = suf.as_str();
+            if looks_like_width_suffix(&['i', 'u'], &suf) {
+                // If it looks like a width, try to be helpful.
+                sess.emit_err(InvalidIntLiteralWidth { span, width: suf[1..].into() });
+            } else if let Some(fixed) = fix_base_capitalisation(suf) {
+                sess.emit_err(InvalidNumLiteralBasePrefix { span, fixed });
+            } else {
+                sess.emit_err(InvalidNumLiteralSuffix { span, suffix: suf.to_string() });
+            }
+        }
+        LitError::InvalidFloatSuffix => {
+            let suf = suffix.expect("suffix error with no suffix");
+            let suf = suf.as_str();
+            if looks_like_width_suffix(&['f'], suf) {
+                // If it looks like a width, try to be helpful.
+                sess.emit_err(InvalidFloatLiteralWidth { span, width: suf[1..].to_string() });
+            } else {
+                sess.emit_err(InvalidFloatLiteralSuffix { span, suffix: suf.to_string() });
+            }
+        }
+        LitError::NonDecimalFloat(base) => {
+            match base {
+                16 => sess.emit_err(HexadecimalFloatLiteralNotSupported { span }),
+                8 => sess.emit_err(OctalFloatLiteralNotSupported { span }),
+                2 => sess.emit_err(BinaryFloatLiteralNotSupported { span }),
+                _ => unreachable!(),
+            };
+        }
+        LitError::IntTooLarge => {
+            sess.emit_err(IntLiteralTooLarge { span });
+        }
+    }
+}