Auto merge of #10676 - djmcgill:origin/master, r=weihanglo

add validation for string "true"/"false" in lto profile

### What does this PR try to resolve?
Adds a special-cased error message for when `lto` is set to the _string_ `"true"`/`"false"` which is surprisingly (I was surprised anyway) not allowed and the error message is ambiguous. The new error message makes it clear what values are accepted.
Fixes #10572

### How should we test and review this PR?

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Uh I've not actually tested yet that's the WIP part. But put
```
[profile.dev]
lto="false"
```
in your TOML and run `cargo build`, check that you get the new error message and that it makes sense and is helpful.

### Additional information

It's worth noting that as per rust-lang/rust#97051 this doesn't fix the _real_ problem here IMO which is that [rust's `opt_parse_bool` cli parsing](https://github.com/rust-lang/rust/blob/491f619f564a4ff9ae4cc837e27bb919d04c31be/compiler/rustc_session/src/options.rs#L456) doesn't accept true/false which certainly seems an ad-hoc historical choice to me on first glance but also it's a much bigger change to change those semantics than this error message.

Author: bors

src/cargo/util/toml/mod.rs

@@ -655,6 +655,16 @@ impl TomlProfile {
             }
         }
 
+        if let Some(StringOrBool::String(arg)) = &self.lto {
+            if arg == "true" || arg == "false" {
+                bail!(
+                    "`lto` setting of string `\"{arg}\"` for `{name}` profile is not \
+                     a valid setting, must be a boolean (`true`/`false`) or a string \
+                    (`\"thin\"`/`\"fat\"`/`\"off\"`) or omitted.",
+                );
+            }
+        }
+
         Ok(())
     }
 

tests/testsuite/profiles.rs

@@ -323,6 +323,37 @@ fn profile_in_virtual_manifest_works() {
         .run();
 }
 
+#[cargo_test]
+fn profile_lto_string_bool_dev() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.0.1"
+
+                [profile.dev]
+                lto = "true"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("build")
+        .with_status(101)
+        .with_stderr(
+            "\
+error: failed to parse manifest at `[ROOT]/foo/Cargo.toml`
+
+Caused by:
+  `lto` setting of string `\"true\"` for `dev` profile is not a valid setting, \
+must be a boolean (`true`/`false`) or a string (`\"thin\"`/`\"fat\"`/`\"off\"`) or omitted.
+",
+        )
+        .run();
+}
+
 #[cargo_test]
 fn profile_panic_test_bench() {
     let p = project()