Rollup merge of #84717 - dtolnay:literalfromstr, r=petrochenkov

impl FromStr for proc_macro::Literal

Note that unlike `impl FromStr for proc_macro::TokenStream`, this impl does not permit whitespace or comments. The input string must consist of nothing but your literal.

- `"1".parse::<Literal>()` ⟶ ok

- `"1.0".parse::<Literal>()` ⟶ ok

- `"'a'".parse::<Literal>()` ⟶ ok

- `"\"\n\"".parse::<Literal>()` ⟶ ok

- `"0 1".parse::<Literal>()` ⟶ LexError

- `" 0".parse::<Literal>()` ⟶ LexError

- `"0 ".parse::<Literal>()` ⟶ LexError

- `"/* comment */0".parse::<Literal>()` ⟶ LexError

- `"0/* comment */".parse::<Literal>()` ⟶ LexError

- `"0// comment".parse::<Literal>()` ⟶ LexError

---

## Use case

```rust
let hex_int: Literal = format!("0x{:x}", int).parse().unwrap();
```

The only way this is expressible in the current API is significantly worse.

```rust
let hex_int = match format!("0x{:x}", int)
    .parse::<TokenStream>()
    .unwrap()
    .into_iter()
    .next()
    .unwrap()
{
    TokenTree::Literal(literal) => literal,
    _ => unreachable!(),
};
```

4 files changed, 80 insertions, 0 deletions

diff --git a/src/test/ui/proc-macro/auxiliary/api/cmp.rs b/src/test/ui/proc-macro/auxiliary/api/cmp.rs
new file mode 100644
index 00000000000..5784a6e5d94
--- /dev/null
+++ b/src/test/ui/proc-macro/auxiliary/api/cmp.rs
@@ -0,0 +1,21 @@
+use proc_macro::{LineColumn, Punct, Spacing};
+
+pub fn test() {
+    test_line_column_ord();
+    test_punct_eq();
+}
+
+fn test_line_column_ord() {
+    let line0_column0 = LineColumn { line: 0, column: 0 };
+    let line0_column1 = LineColumn { line: 0, column: 1 };
+    let line1_column0 = LineColumn { line: 1, column: 0 };
+    assert!(line0_column0 < line0_column1);
+    assert!(line0_column1 < line1_column0);
+}
+
+fn test_punct_eq() {
+    let colon_alone = Punct::new(':', Spacing::Alone);
+    assert_eq!(colon_alone, ':');
+    let colon_joint = Punct::new(':', Spacing::Joint);
+    assert_eq!(colon_joint, ':');
+}
diff --git a/src/test/ui/proc-macro/auxiliary/api/mod.rs b/src/test/ui/proc-macro/auxiliary/api/mod.rs
new file mode 100644
index 00000000000..739c25132e7
--- /dev/null
+++ b/src/test/ui/proc-macro/auxiliary/api/mod.rs
@@ -0,0 +1,24 @@
+// force-host
+// no-prefer-dynamic
+
+#![crate_type = "proc-macro"]
+#![crate_name = "proc_macro_api_tests"]
+#![feature(proc_macro_span)]
+#![deny(dead_code)] // catch if a test function is never called
+
+extern crate proc_macro;
+
+mod cmp;
+mod parse;
+
+use proc_macro::TokenStream;
+
+#[proc_macro]
+pub fn run(input: TokenStream) -> TokenStream {
+    assert!(input.is_empty());
+
+    cmp::test();
+    parse::test();
+
+    TokenStream::new()
+}
diff --git a/src/test/ui/proc-macro/auxiliary/api/parse.rs b/src/test/ui/proc-macro/auxiliary/api/parse.rs
new file mode 100644
index 00000000000..4105236b7f2
--- /dev/null
+++ b/src/test/ui/proc-macro/auxiliary/api/parse.rs
@@ -0,0 +1,23 @@
+use proc_macro::Literal;
+
+pub fn test() {
+    test_parse_literal();
+}
+
+fn test_parse_literal() {
+    assert_eq!("1".parse::<Literal>().unwrap().to_string(), "1");
+    assert_eq!("1.0".parse::<Literal>().unwrap().to_string(), "1.0");
+    assert_eq!("'a'".parse::<Literal>().unwrap().to_string(), "'a'");
+    assert_eq!("\"\n\"".parse::<Literal>().unwrap().to_string(), "\"\n\"");
+    assert_eq!("b\"\"".parse::<Literal>().unwrap().to_string(), "b\"\"");
+    assert_eq!("r##\"\"##".parse::<Literal>().unwrap().to_string(), "r##\"\"##");
+    assert_eq!("10ulong".parse::<Literal>().unwrap().to_string(), "10ulong");
+
+    assert!("0 1".parse::<Literal>().is_err());
+    assert!("'a".parse::<Literal>().is_err());
+    assert!(" 0".parse::<Literal>().is_err());
+    assert!("0 ".parse::<Literal>().is_err());
+    assert!("/* comment */0".parse::<Literal>().is_err());
+    assert!("0/* comment */".parse::<Literal>().is_err());
+    assert!("0// comment".parse::<Literal>().is_err());
+}
diff --git a/src/test/ui/proc-macro/test.rs b/src/test/ui/proc-macro/test.rs
new file mode 100644
index 00000000000..c96aa73175f
--- /dev/null
+++ b/src/test/ui/proc-macro/test.rs
@@ -0,0 +1,12 @@
+// check-pass
+// aux-build:api/mod.rs
+
+//! This is for everything that *would* be a #[test] inside of libproc_macro,
+//! except for the fact that proc_macro objects are not capable of existing
+//! inside of an ordinary Rust test execution, only inside a macro.
+
+extern crate proc_macro_api_tests;
+
+proc_macro_api_tests::run!();
+
+fn main() {}