Fix overflow panic in convert_interger_literal assist

This also seizes the opportunity to move integer literal parsing to
the syntax crate, were it logically belongs.

Note though that this is still done in an ad hoc manner -- we probably
should split kitchen sink ast::Literal into a separate APIs for
strings, ints, etc

crates/syntax/src/ast/expr_ext.rs

@@ -358,6 +358,71 @@ impl ast::Literal {
             _ => unreachable!(),
         }
     }
+
+    // FIXME: should probably introduce string token type?
+    // https://github.com/rust-analyzer/rust-analyzer/issues/6308
+    pub fn int_value(&self) -> Option<(Radix, u128)> {
+        let suffix = match self.kind() {
+            LiteralKind::IntNumber { suffix } => suffix,
+            _ => return None,
+        };
+
+        let token = self.token();
+        let mut text = token.text().as_str();
+        text = &text[..text.len() - suffix.map_or(0, |it| it.len())];
+
+        let buf;
+        if text.contains("_") {
+            buf = text.replace('_', "");
+            text = buf.as_str();
+        };
+
+        let radix = Radix::identify(text)?;
+        let digits = &text[radix.prefix_len()..];
+        let value = u128::from_str_radix(digits, radix as u32).ok()?;
+        Some((radix, value))
+    }
+}
+
+#[derive(Debug, PartialEq, Eq, Copy, Clone)]
+pub enum Radix {
+    Binary = 2,
+    Octal = 8,
+    Decimal = 10,
+    Hexadecimal = 16,
+}
+
+impl Radix {
+    pub const ALL: &'static [Radix] =
+        &[Radix::Binary, Radix::Octal, Radix::Decimal, Radix::Hexadecimal];
+
+    fn identify(literal_text: &str) -> Option<Self> {
+        // We cannot express a literal in anything other than decimal in under 3 characters, so we return here if possible.
+        if literal_text.len() < 3 && literal_text.chars().all(|c| c.is_digit(10)) {
+            return Some(Self::Decimal);
+        }
+
+        let res = match &literal_text[..2] {
+            "0b" => Radix::Binary,
+            "0o" => Radix::Octal,
+            "0x" => Radix::Hexadecimal,
+            _ => Radix::Decimal,
+        };
+
+        // Checks that all characters after the base prefix are all valid digits for that base.
+        if literal_text[res.prefix_len()..].chars().all(|c| c.is_digit(res as u32)) {
+            Some(res)
+        } else {
+            None
+        }
+    }
+
+    const fn prefix_len(&self) -> usize {
+        match self {
+            Self::Decimal => 0,
+            _ => 2,
+        }
+    }
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]